
# Tactics¶

Tactics specify how to transform the proof state of an incomplete proof to eventually generate a complete proof.

Proofs can be developed in two basic ways: In forward reasoning, the proof begins by proving simple statements that are then combined to prove the theorem statement as the last step of the proof. With forward reasoning, for example, the proof of A /\ B would begin with proofs of A and B, which are then used to prove A /\ B. Forward reasoning is probably the most common approach in human-generated proofs.

In backward reasoning, the proof begins with the theorem statement as the goal, which is then gradually transformed until every subgoal generated along the way has been proven. In this case, the proof of A /\ B begins with that formula as the goal. This can be transformed into two subgoals, A and B, followed by the proofs of A and B. Coq and its tactics use backward reasoning.

A tactic may fully prove a goal, in which case the goal is removed from the proof state. More commonly, a tactic replaces a goal with one or more subgoals. (We say that a tactic reduces a goal to its subgoals.)

Most tactics require specific elements or preconditions to reduce a goal; they display error messages if they can't be applied to the goal. A few tactics, such as auto, don't fail even if the proof state is unchanged.

Goals are identified by number. The current goal is number 1. Tactics are applied to the current goal by default. (The default can be changed with the Default Goal Selector option.) They can be applied to another goal or to multiple goals with a goal selector such as 2: tactic.

This chapter describes many of the most common built-in tactics. Built-in tactics can be combined to form tactic expressions, which are described in the Ltac chapter. Since tactic expressions can be used anywhere that a built-in tactic can be used, "tactic" may refer to both built-in tactics and tactic expressions.

## Common elements of tactics¶

### Reserved keywords¶

The tactics described in this chapter reserve the following keywords:

by using


Thus, these keywords cannot be used as identifiers. It also declares the following character sequences as tokens:

** [= |-


### Invocation of tactics¶

A tactic is applied as an ordinary command. It may be preceded by a goal selector (see Section Goal selectors). If no selector is specified, the default selector is used.

::=|
Option Default Goal Selector "toplevel_selector"

This option controls the default selector, used when no selector is specified when applying a tactic. The initial value is 1, hence the tactics are, by default, applied to the first goal.

Using value all will make it so that tactics are, by default, applied to every goal simultaneously. Then, to apply a tactic tac to the first goal only, you can write 1:tac.

Using value ! enforces that all tactics are used either on a single focused goal or with a local selector (’’strict focusing mode’’).

Although other selectors are available, only all, ! or a single natural number are valid default goal selectors.

### Bindings¶

Tactics that take a term as an argument may also accept bindings to instantiate some parameters of the term by name or position. The general form of a term with bindings is termtac with bindings where bindings can take two different forms:

::=( := term )+|

### Intro patterns¶

Intro patterns let you specify the name to assign to variables and hypotheses introduced by tactics. They also let you split an introduced hypothesis into multiple hypotheses or subgoals. Common tactics that accept intro patterns include assert, intros and destruct.

::=*|**|simple_intropattern::=::=naming_intropattern|_|or_and_intropattern|equality_intropattern::=ident|?|::=[ *| ]||::=->|<-|[= ]

Note that the intro pattern syntax varies between tactics. Most tactics use simple_intropattern in the grammar. destruct, edestruct, induction, einduction, case, ecase and the various inversion tactics use or_and_intropattern, while intros and eintros use intropattern*. The eqn: construct in various tactics uses naming_intropattern.

Naming patterns

Use these elementary patterns to specify a name:

• ident — use the specified name

• ? — let Coq generate a fresh name

• ?ident — generate a name that begins with ident

• _ — discard the matched part (unless it is required for another hypothesis)

• if a disjunction pattern omits a name, such as [|H2], Coq will choose a name

Splitting patterns

The most common splitting patterns are:

• split a hypothesis in the form A /\ B into two hypotheses H1: A and H2: B using the pattern (H1 & H2) or (H1, H2) or [H1 H2]. Example. This also works on A <-> B, which is just a notation representing (A -> B) /\ (B -> A).

• split a hypothesis in the form A \/ B into two subgoals using the pattern [H1|H2]. The first subgoal will have the hypothesis H1: A and the second subgoal will have the hypothesis H2: B. Example

• split a hypothesis in either of the forms A /\ B or A \/ B using the pattern [].

Patterns can be nested: [[Ha|Hb] H] can be used to split (A \/ B) /\ C.

Note that there is no equivalent to intro patterns for goals. For a goal A /\ B, use the split tactic to replace the current goal with subgoals A and B. For a goal A \/ B, use left to replace the current goal with A, or right to replace the current goal with B.

• ( simple_intropattern+, ) — matches a product over an inductive type with a single constructor. If the number of patterns equals the number of constructor arguments, then it applies the patterns only to the arguments, and ( simple_intropattern+, ) is equivalent to [simple_intropattern+]. If the number of patterns equals the number of constructor arguments plus the number of let-ins, the patterns are applied to the arguments and let-in variables.

• ( simple_intropattern+& ) — matches a right-hand nested term that consists of one or more nested binary inductive types such as a1 OP1 a2 OP2 ... (where the OPn are right-associative). (If the OPn are left-associative, additional parentheses will be needed to make the term right-hand nested, such as a1 OP1 (a2 OP2 ...).) The splitting pattern can have more than 2 names, for example (H1 & H2 & H3) matches A /\ B /\ C. The inductive types must have a single constructor with two parameters. Example

• [ intropattern*+| ] — splits an inductive type that has multiple constructors such as A \/ B into multiple subgoals. The number of intropatterns must be the same as the number of constructors for the matched part.

• [ intropattern+ ] — splits an inductive type that has a single constructor with multiple parameters such as A /\ B into multiple hypotheses. Use [H1 [H2 H3]] to match A /\ B /\ C.

• [] — splits an inductive type: If the inductive type has multiple constructors, such as A \/ B, create one subgoal for each constructor. If the inductive type has a single constructor with multiple parameters, such as A /\ B, split it into multiple hypotheses.

Equality patterns

These patterns can be used when the hypothesis is an equality:

• -> — replaces the right-hand side of the hypothesis with the left-hand side of the hypothesis in the conclusion of the goal; the hypothesis is cleared; if the left-hand side of the hypothesis is a variable, it is substituted everywhere in the context and the variable is removed. Example

• <- — similar to ->, but replaces the left-hand side of the hypothesis with the right-hand side of the hypothesis.

• [= intropattern*, ] — If the product is over an equality type, applies either injection or discriminate. If injection is applicable, the intropattern is used on the hypotheses generated by injection. If the number of patterns is smaller than the number of hypotheses generated, the pattern ? is used to complete the list. Example

Other patterns

• * — introduces one or more quantified variables from the result until there are no more quantified variables. Example

• ** — introduces one or more quantified variables or hypotheses from the result until there are no more quantified variables or implications (->). intros ** is equivalent to intros. Example

• simple_intropattern_closed % term* — first applies each of the terms with the apply … in tactic on the hypothesis to be introduced, then it uses simple_intropattern_closed. Example

Note

A \/ B and A /\ B use infix notation to refer to the inductive types or and and. or has multiple constructors (or_introl and or_intror), while and has a single constructor (conj) with multiple parameters (A and B). These are defined in theories/Init/Logic.v. The "where" clauses define the infix notation for "or" and "and".

Inductive or (A B:Prop) : Prop :=   | or_introl : A -> A \/ B   | or_intror : B -> A \/ B where "A \/ B" := (or A B) : type_scope. Inductive and (A B:Prop) : Prop :=   conj : A -> B -> A /\ B where "A /\ B" := (and A B) : type_scope.

Note

intros p+ is not always equivalent to intros p; ... ; intros p if some of the p are _. In the first form, all erasures are done at once, while they're done sequentially for each tactic in the second form. If the second matched term depends on the first matched term and the pattern for both is _ (i.e., both will be erased), the first intros in the second form will fail because the second matched term still has the dependency on the first.

Examples:

Example: intro pattern for /\

Goal forall (A: Prop) (B: Prop), (A /\ B) -> True.
1 goal ============================ forall A B : Prop, A /\ B -> True
intros.
1 goal A, B : Prop H : A /\ B ============================ True
destruct H as (HA & HB).
1 goal A, B : Prop HA : A HB : B ============================ True

Example: intro pattern for \/

Goal forall (A: Prop) (B: Prop), (A \/ B) -> True.
1 goal ============================ forall A B : Prop, A \/ B -> True
intros.
1 goal A, B : Prop H : A \/ B ============================ True
destruct H as [HA|HB]. all: swap 1 2.
2 goals A, B : Prop HA : A ============================ True goal 2 is: True 2 goals A, B : Prop HB : B ============================ True goal 2 is: True

Example: -> intro pattern

Goal forall (x:nat) (y:nat) (z:nat), (x = y) -> (y = z) -> (x = z).
1 goal ============================ forall x y z : nat, x = y -> y = z -> x = z
intros * H.
1 goal x, y, z : nat H : x = y ============================ y = z -> x = z
intros ->.
1 goal x, z : nat H : x = z ============================ x = z

Example: [=] intro pattern

The first intros [=] uses injection to strip (S ...) from both sides of the matched equality. The second uses discriminate on the contradiction 1 = 2 (internally represented as (S O) = (S (S O))) to complete the goal.

Goal forall (n m:nat), (S n) = (S m) -> (S O)=(S (S O)) -> False.
1 goal ============================ forall n m : nat, S n = S m -> 1 = 2 -> False
intros *.
1 goal n, m : nat ============================ S n = S m -> 1 = 2 -> False
intros [= H].
1 goal n, m : nat H : n = m ============================ 1 = 2 -> False
intros [=].
No more goals.

Example: (A & B & ...) intro pattern

Parameters (A : Prop) (B: nat -> Prop) (C: Prop).
A is declared B is declared C is declared
Goal A /\ (exists x:nat, B x /\ C) -> True.
1 goal ============================ A /\ (exists x : nat, B x /\ C) -> True
intros (a & x & b & c).
1 goal a : A x : nat b : B x c : C ============================ True

Example: * intro pattern

Goal forall (A: Prop) (B: Prop), A -> B.
1 goal ============================ forall A B : Prop, A -> B
intros *.
1 goal A, B : Prop ============================ A -> B

Example: ** pattern ("intros **" is equivalent to "intros")

Goal forall (A: Prop) (B: Prop), A -> B.
1 goal ============================ forall A B : Prop, A -> B
intros **.
1 goal A, B : Prop H : A ============================ B

Example: compound intro pattern

Goal forall A B C:Prop, A \/ B /\ C -> (A -> C) -> C.
1 goal ============================ forall A B C : Prop, A \/ B /\ C -> (A -> C) -> C
intros * [a | (_,c)] f.
2 goals A, B, C : Prop a : A f : A -> C ============================ C goal 2 is: C
all: swap 1 2.
2 goals A, B, C : Prop c : C f : A -> C ============================ C goal 2 is: C

Example: combined intro pattern using [=] -> and %

Require Import Coq.Lists.List.
Section IntroPatterns.
Variables (A : Type) (xs ys : list A).
A is declared xs is declared ys is declared
Example ThreeIntroPatternsCombined : S (length ys) = 1 -> xs ++ ys = xs.
1 goal A : Type xs, ys : list A ============================ S (length ys) = 1 -> xs ++ ys = xs
intros [=->%length_zero_iff_nil].
1 goal A : Type xs : list A ============================ xs ++ nil = xs
• intros would add H : S (length ys) = 1

• intros [=] would additionally apply injection to H to yield H0 : length ys = 0

• intros [=->%length_zero_iff_nil] applies the theorem, making H the equality l=nil, which is then applied as for ->.

Theorem length_zero_iff_nil (l : list A):    length l = 0 <-> l=nil.

The example is based on Tej Chajed's coq-tricks

### Occurrence clauses¶

An occurrence is a subterm of a goal or hypothesis that matches a pattern provided by a tactic. Occurrence clauses select a subset of the ocurrences in a goal and/or in one or more of its hypotheses.

::=|::=occurrences::=-? ::=::=hyp_occs+, |- ?|* |- ||- |::=::=ident|( type of ident )|( value of ident )::=* ?
occurrences

The first form of occurrences selects occurrences in the conclusion of the goal. The second form can select occurrences in the goal conclusion and in one or more hypotheses.

simple_occurrences

A semantically restricted form of occurrences that doesn't allow the at clause anywhere within it.

-? nat_or_var+

Selects the specified occurrences within a single goal or hypothesis. Occurrences are numbered starting with 1 following a depth-first traversal of the term's expression, including occurrences in implicit arguments and coercions that are not displayed by default. (Set the Printing All flag to show those in the printed term.)

For example, when matching the pattern _ + _ in the term (a + b) + c, occurrence 1 is (...) + c and occurrence 2 is (a + b). When matching that pattern with term a + (b + c), occurrence 1 is a + (...) and occurrence 2 is b + c.

Specifying - includes all occurrences except the ones listed.

hyp_occs*, |- concl_occs??

Selects occurrences in the specified hypotheses and the specified occurrences in the conclusion.

* |- concl_occs?

Selects all occurrences in all hypotheses and the specified occurrences in the conclusion.

|- concl_occs?

Selects the specified occurrences in the conclusion.

goal_occurrences ::= concl_occs?

Selects all occurrences in all hypotheses and in the specified occurrences in the conclusion.

hypident at occs_nums?

Omiting occs_nums selects all occurrences within the hypothesis.

hypident ::= ident

Selects the hypothesis named ident.

( type of ident )

Selects the type part of the named hypothesis (e.g. : nat).

( value of ident )

Selects the value part of the named hypothesis (e.g. := 1).

concl_occs ::= * at occs_nums?

Selects occurrences in the conclusion. '*' by itself selects all occurrences. occs_nums selects the specified occurrences.

Use in * to select all occurrences in all hypotheses and the conclusion, which is equivalent to in * |- *. Use * |- to select all occurrences in all hypotheses.

Tactics that select a specific hypothesis H to apply to other hypotheses, such as rewrite H in * |-, won't apply H to itself.

If multiple occurrences are given, such as in rewrite H at 1 2 3, the tactic must match at least one occurrence in order to succeed. The tactic will fail if no occurrences match. Occurrence numbers that are out of range (e.g. at 1 3 when there are only 2 occurrences in the hypothesis or conclusion) are ignored.

Tactics that use occurrence clauses include set, remember, induction and destruct.

## Applying theorems¶

Tactic exact term

This tactic applies to any goal. It gives directly the exact proof term of the goal. Let T be our goal, let p be a term of type U then exact p succeeds iff T and U are convertible (see Conversion rules).

Error Not an exact proof.
Variant eexact term.

This tactic behaves like exact but is able to handle terms and goals with existential variables.

Tactic assumption

This tactic looks in the local context for a hypothesis whose type is convertible to the goal. If it is the case, the subgoal is proved. Otherwise, it fails.

Error No such assumption.
Variant eassumption

This tactic behaves like assumption but is able to handle goals with existential variables.

Tactic refine term

This tactic applies to any goal. It behaves like exact with a big difference: the user can leave some holes (denoted by _ or (_ : type)) in the term. refine will generate as many subgoals as there are remaining holes in the elaborated term. The type of holes must be either synthesized by the system or declared by an explicit cast like (_ : nat -> Prop). Any subgoal that occurs in other subgoals is automatically shelved, as if calling shelve_unifiable. The produced subgoals (shelved or not) are not candidates for typeclass resolution, even if they have a type-class type as conclusion, letting the user control when and how typeclass resolution is launched on them. This low-level tactic can be useful to advanced users.

Example

Inductive Option : Set := | Fail : Option | Ok : bool -> Option.
Option is defined Option_rect is defined Option_ind is defined Option_rec is defined Option_sind is defined
Definition get : forall x:Option, x <> Fail -> bool.
1 goal ============================ forall x : Option, x <> Fail -> bool
refine     (fun x:Option =>       match x return x <> Fail -> bool with       | Fail => _       | Ok b => fun _ => b       end).
1 goal x : Option ============================ Fail <> Fail -> bool
intros; absurd (Fail = Fail); trivial.
No more goals.
Defined.
Error Invalid argument.

The tactic refine does not know what to do with the term you gave.

Error Refine passed ill-formed term.

The term you gave is not a valid proof (not easy to debug in general). This message may also occur in higher-level tactics that call refine internally.

Error Cannot infer a term for this placeholder.

There is a hole in the term you gave whose type cannot be inferred. Put a cast around it.

Variant simple refine term

This tactic behaves like refine, but it does not shelve any subgoal. It does not perform any beta-reduction either.

Variant notypeclasses refine term

This tactic behaves like refine except it performs type checking without resolution of typeclasses.

Variant simple notypeclasses refine term

This tactic behaves like the combination of simple refine and notypeclasses refine: it performs type checking without resolution of typeclasses, does not perform beta reductions or shelve the subgoals.

Debug "unification" enables printing traces of unification steps used during elaboration/typechecking and the refine tactic. "ho-unification" prints information about higher order heuristics.

Tactic apply term

This tactic applies to any goal. The argument term is a term well-formed in the local context. The tactic apply tries to match the current goal against the conclusion of the type of term. If it succeeds, then the tactic returns as many subgoals as the number of non-dependent premises of the type of term. If the conclusion of the type of term does not match the goal and the conclusion is an inductive type isomorphic to a tuple type, then each component of the tuple is recursively matched to the goal in the left-to-right order.

The tactic apply relies on first-order unification with dependent types unless the conclusion of the type of term is of the form P (t1 ... tn) with P to be instantiated. In the latter case, the behavior depends on the form of the goal. If the goal is of the form (fun x => Q) u1 ... un and the ti and ui unify, then P is taken to be (fun x => Q). Otherwise, apply tries to define P by abstracting over t_1 ... t__n in the goal. See pattern to transform the goal so that it gets the form (fun x => Q) u1 ... un.

Error Unable to unify term with term.

The apply tactic failed to match the conclusion of term and the current goal. You can help the apply tactic by transforming your goal with the change or pattern tactics.

Error Unable to find an instance for the variables ident+.

This occurs when some instantiations of the premises of term are not deducible from the unification. This is the case, for instance, when you want to apply a transitivity property. In this case, you have to use one of the variants below:

Variant apply term with term+

Provides apply with explicit instantiations for all dependent premises of the type of term that do not occur in the conclusion and consequently cannot be found by unification. Notice that the collection term+ must be given according to the order of these dependent premises of the type of term.

Error Not the right number of missing arguments.
Variant apply term with bindings

This also provides apply with values for instantiating premises. Here, variables are referred by names and non-dependent products by increasing numbers (see Bindings).

Variant apply term+,

This is a shortcut for apply term1; [.. | ... ; [ .. | apply termn] ... ], i.e. for the successive applications of termi+1 on the last subgoal generated by apply termi , starting from the application of term1.

Variant eapply term

The tactic eapply behaves like apply but it does not fail when no instantiations are deducible for some variables in the premises. Rather, it turns these variables into existential variables which are variables still to instantiate (see Existential variables). The instantiation is intended to be found later in the proof.

Variant rapply term

The tactic rapply behaves like eapply but it uses the proof engine of refine for dealing with existential variables, holes, and conversion problems. This may result in slightly different behavior regarding which conversion problems are solvable. However, like apply but unlike eapply, rapply will fail if there are any holes which remain in term itself after typechecking and typeclass resolution but before unification with the goal. More technically, term is first parsed as a constr rather than as a uconstr or open_constr before being applied to the goal. Note that rapply prefers to instantiate as many hypotheses of term as possible. As a result, if it is possible to apply term to arbitrarily many arguments without getting a type error, rapply will loop.

Note that you need to Require Import Coq.Program.Tactics to make use of rapply.

Variant simple apply term.

This behaves like apply but it reasons modulo conversion only on subterms that contain no variables to instantiate. For instance, the following example does not succeed because it would require the conversion of id ?foo and O.

Example

Definition id (x : nat) := x.
id is defined
Parameter H : forall x y, id x = y.
H is declared
Goal O = O.
1 goal ============================ 0 = 0
Fail simple apply H.
The command has indeed failed with message: Unable to unify "id ?M160 = ?M161" with "0 = 0".

Because it reasons modulo a limited amount of conversion, simple apply fails quicker than apply and it is then well-suited for uses in user-defined tactics that backtrack often. Moreover, it does not traverse tuples as apply does.

Variant simple? apply term with bindings?+,
Variant simple? eapply term with bindings?+,

This summarizes the different syntaxes for apply and eapply.

Variant lapply term

This tactic applies to any goal, say G. The argument term has to be well-formed in the current context, its type being reducible to a non-dependent product A -> B with B possibly containing products. Then it generates two subgoals B->G and A. Applying lapply H (where H has type A->B and B does not start with a product) does the same as giving the sequence cut B. 2:apply H. where cut is described below.

Example

Assume we have a transitive relation R on nat:

Parameter R : nat -> nat -> Prop.
R is declared
Axiom Rtrans : forall x y z:nat, R x y -> R y z -> R x z.
Rtrans is declared
Parameters n m p : nat.
n is declared m is declared p is declared
Axiom Rnm : R n m.
Rnm is declared
Axiom Rmp : R m p.
Rmp is declared

Consider the goal (R n p) provable using the transitivity of R:

Goal R n p.
1 goal ============================ R n p

The direct application of Rtrans with apply fails because no value for y in Rtrans is found by apply:

apply Rtrans.
Toplevel input, characters 6-12: > apply Rtrans. > ^^^^^^ Error: Unable to find an instance for the variable y.

A solution is to apply (Rtrans n m p) or (Rtrans n m).

apply (Rtrans n m p).
2 goals ============================ R n m goal 2 is: R m p

Note that n can be inferred from the goal, so the following would work too.

apply (Rtrans _ m).
2 goals ============================ R n m goal 2 is: R m p

More elegantly, apply Rtrans with (y:=m) allows only mentioning the unknown m:

apply Rtrans with (y := m).
2 goals ============================ R n m goal 2 is: R m p

Another solution is to mention the proof of (R x y) in Rtrans

apply Rtrans with (1 := Rnm).
1 goal ============================ R m p

... or the proof of (R y z).

apply Rtrans with (2 := Rmp).
1 goal ============================ R n m

On the opposite, one can use eapply which postpones the problem of finding m. Then one can apply the hypotheses Rnm and Rmp. This instantiates the existential variable and completes the proof.

eapply Rtrans.
2 focused goals (shelved: 1) ============================ R n ?y goal 2 is: R ?y p
apply Rnm.
1 goal ============================ R m p
apply Rmp.
No more goals.

Note

When the conclusion of the type of the term to apply is an inductive type isomorphic to a tuple type and apply looks recursively whether a component of the tuple matches the goal, it excludes components whose statement would result in applying an universal lemma of the form forall A, ... -> A. Excluding this kind of lemma can be avoided by setting the following flag:

Flag Universal Lemma Under Conjunction

This flag, which preserves compatibility with versions of Coq prior to 8.4 is also available for apply term in ident (see apply … in).

Tactic apply term in ident

This tactic applies to any goal. The argument term is a term well-formed in the local context and the argument ident is an hypothesis of the context. The tactic apply term in ident tries to match the conclusion of the type of ident against a non-dependent premise of the type of term, trying them from right to left. If it succeeds, the statement of hypothesis ident is replaced by the conclusion of the type of term. The tactic also returns as many subgoals as the number of other non-dependent premises in the type of term and of the non-dependent premises of the type of ident. If the conclusion of the type of term does not match the goal and the conclusion is an inductive type isomorphic to a tuple type, then the tuple is (recursively) decomposed and the first component of the tuple of which a non-dependent premise matches the conclusion of the type of ident. Tuples are decomposed in a width-first left-to-right order (for instance if the type of H1 is A <-> B and the type of H2 is A then apply H1 in H2 transforms the type of H2 into B). The tactic apply relies on first-order pattern matching with dependent types.

Error Statement without assumptions.

This happens if the type of term has no non-dependent premise.

Error Unable to apply.

This happens if the conclusion of ident does not match any of the non-dependent premises of the type of term.

Variant apply term+, in ident+,

This applies each term in sequence in each hypothesis ident.

Variant apply term with bindings+, in ident+,

This does the same but uses the bindings to instantiate parameters of term (see Bindings).

Variant eapply term with bindings?+, in ident+,

This works as apply … in but turns unresolved bindings into existential variables, if any, instead of failing.

Variant apply term with bindings?+, in ident as simple_intropattern?+,

This works as apply … in but applying an associated simple_intropattern to each hypothesis ident that comes with such clause.

Variant simple apply term in ident+,

This behaves like apply … in but it reasons modulo conversion only on subterms that contain no variables to instantiate and does not traverse tuples. See the corresponding example.

Variant simple? apply term with bindings?+, in ident as simple_intropattern?+,
Variant simple? eapply term with bindings?+, in ident as simple_intropattern?+,

This summarizes the different syntactic variants of apply term in ident+, and eapply term in ident+,.

Debug "tactic-unification" enables printing traces of unification steps in tactic unification. Tactic unification is used in tactics such as apply and rewrite.

## Managing the local context¶

Tactic intro

This tactic applies to a goal that is either a product or starts with a let-binder. If the goal is a product, the tactic implements the "Lam" rule given in Typing rules 1. If the goal starts with a let-binder, then the tactic implements a mix of the "Let" and "Conv".

If the current goal is a dependent product forall x:T, U (resp let x:=t in U) then intro puts x:T (resp x:=t) in the local context. The new subgoal is U.

If the goal is a non-dependent product $$T \rightarrow U$$, then it puts in the local context either Hn:T (if T is of type Set or Prop) or Xn:T (if the type of T is Type). The optional index n is such that Hn or Xn is a fresh identifier. In both cases, the new subgoal is U.

If the goal is an existential variable, intro forces the resolution of the existential variable into a dependent product $$\forall$$ x:?X, ?Y, puts x:?X in the local context and leaves ?Y as a new subgoal allowed to depend on x.

The tactic intro applies the tactic hnf until intro can be applied or the goal is not head-reducible.

Error No product even after head-reduction.
Variant intro ident

This applies intro but forces ident to be the name of the introduced hypothesis.

Error ident is already used.

Note

If a name used by intro hides the base name of a global constant then the latter can still be referred to by a qualified name (see Qualified identifiers).

Variant intros

This repeats intro until it meets the head-constant. It never reduces head-constants and it never fails.

Variant intros ident+.

This is equivalent to the composed tactic intro ident; ... ; intro ident.

Variant intros until ident

This repeats intro until it meets a premise of the goal having the form (ident : type) and discharges the variable named ident of the current goal.

Error No such hypothesis in current goal.
Variant intros until natural

This repeats intro until the natural-th non-dependent product.

Example

On the subgoal forall x y : nat, x = y -> y = x the tactic intros until 1 is equivalent to intros x y H, as x = y -> y = x is the first non-dependent product.

On the subgoal forall x y z : nat, x = y -> y = x the tactic intros until 1 is equivalent to intros x y z as the product on z can be rewritten as a non-dependent product: forall x y : nat, nat -> x = y -> y = x.

Error No such hypothesis in current goal.

This happens when natural is 0 or is greater than the number of non-dependent products of the goal.

Variant intro ident1? after ident2
Variant intro ident1? before ident2
Variant intro ident1? at top
Variant intro ident1? at bottom

These tactics apply intro ident1? and move the freshly introduced hypothesis respectively after the hypothesis ident2, before the hypothesis ident2, at the top of the local context, or at the bottom of the local context. All hypotheses on which the new hypothesis depends are moved too so as to respect the order of dependencies between hypotheses. It is equivalent to intro ident1? followed by the appropriate call to move, move … before …, move … at top, or move … at bottom.

Note

intro at bottom is a synonym for intro with no argument.

Error No such hypothesis: ident.
Tactic intros intropattern*

Introduces one or more variables or hypotheses from the goal by matching the intro patterns. See the description in Intro patterns.

Tactic eintros intropattern*

Works just like intros … except that it creates existential variables for any unresolved variables rather than failing.

Tactic clear ident

This tactic erases the hypothesis named ident in the local context of the current goal. As a consequence, ident is no more displayed and no more usable in the proof development.

Error No such hypothesis.
Error ident is used in the conclusion.
Error ident is used in the hypothesis ident.
Variant clear ident+

This is equivalent to clear ident. ... clear ident.

Variant clear - ident+

This variant clears all the hypotheses except the ones depending in the hypotheses named ident+ and in the goal.

Variant clear

This variants clears all the hypotheses except the ones the goal depends on.

Variant clear dependent ident

This clears the hypothesis ident and all the hypotheses that depend on it.

Variant clearbody ident+

This tactic expects ident+ to be local definitions and clears their respective bodies. In other words, it turns the given definitions into assumptions.

Error ident is not a local definition.
Tactic revert ident+

This applies to any goal with variables ident+. It moves the hypotheses (possibly defined) to the goal, if this respects dependencies. This tactic is the inverse of intro.

Error No such hypothesis.
Error ident1 is used in the hypothesis ident2.
Variant revert dependent ident

This moves to the goal the hypothesis ident and all the hypotheses that depend on it.

Tactic move ident1 after ident2

This moves the hypothesis named ident1 in the local context after the hypothesis named ident2, where “after” is in reference to the direction of the move. The proof term is not changed.

If ident1 comes before ident2 in the order of dependencies, then all the hypotheses between ident1 and ident2 that (possibly indirectly) depend on ident1 are moved too, and all of them are thus moved after ident2 in the order of dependencies.

If ident1 comes after ident2 in the order of dependencies, then all the hypotheses between ident1 and ident2 that (possibly indirectly) occur in the type of ident1 are moved too, and all of them are thus moved before ident2 in the order of dependencies.

Variant move ident1 before ident2

This moves ident1 towards and just before the hypothesis named ident2. As for move, dependencies over ident1 (when ident1 comes before ident2 in the order of dependencies) or in the type of ident1 (when ident1 comes after ident2 in the order of dependencies) are moved too.

Variant move ident at top

This moves ident at the top of the local context (at the beginning of the context).

Variant move ident at bottom

This moves ident at the bottom of the local context (at the end of the context).

Error No such hypothesis.
Error Cannot move ident1 after ident2: it occurs in the type of ident2.
Error Cannot move ident1 after ident2: it depends on ident2.

Example

Goal forall x :nat, x = 0 -> forall z y:nat, y=y-> 0=x.
1 goal ============================ forall x : nat, x = 0 -> nat -> forall y : nat, y = y -> 0 = x
intros x H z y H0.
1 goal x : nat H : x = 0 z, y : nat H0 : y = y ============================ 0 = x
move x after H0.
1 goal z, y : nat H0 : y = y x : nat H : x = 0 ============================ 0 = x
Undo.
1 goal x : nat H : x = 0 z, y : nat H0 : y = y ============================ 0 = x
move x before H0.
1 goal z, y, x : nat H : x = 0 H0 : y = y ============================ 0 = x
Undo.
1 goal x : nat H : x = 0 z, y : nat H0 : y = y ============================ 0 = x
move H0 after H.
1 goal x, y : nat H0 : y = y H : x = 0 z : nat ============================ 0 = x
Undo.
1 goal x : nat H : x = 0 z, y : nat H0 : y = y ============================ 0 = x
move H0 before H.
1 goal x : nat H : x = 0 y : nat H0 : y = y z : nat ============================ 0 = x
Tactic rename ident1 into ident2

This renames hypothesis ident1 into ident2 in the current context. The name of the hypothesis in the proof-term, however, is left unchanged.

Variant rename identi into identj+,

This renames the variables identi into identj in parallel. In particular, the target identifiers may contain identifiers that exist in the source context, as long as the latter are also renamed by the same tactic.

Error No such hypothesis.
Error ident is already used.
Tactic set (ident := term)

This replaces term by ident in the conclusion of the current goal and adds the new definition ident := term to the local context.

If term has holes (i.e. subexpressions of the form “_”), the tactic first checks that all subterms matching the pattern are compatible before doing the replacement using the leftmost subterm matching the pattern.

Error The variable ident is already defined.
Variant set (ident := term) in goal_occurrences

This notation allows specifying which occurrences of term have to be substituted in the context. The in goal_occurrences clause is an occurrence clause whose syntax and behavior are described in goal occurrences.

Variant set (ident binder* := term) in goal_occurrences?

This is equivalent to set (ident := fun binder* => term) in goal_occurrences?.

Variant set term in goal_occurrences?

This behaves as set (ident := term) in goal_occurrences? but ident is generated by Coq.

Variant eset (ident binder* := term) in goal_occurrences?
Variant eset term in goal_occurrences?

While the different variants of set expect that no existential variables are generated by the tactic, eset removes this constraint. In practice, this is relevant only when eset is used as a synonym of epose, i.e. when the term does not occur in the goal.

Tactic remember term as ident1 eqn:naming_intropattern?

This behaves as set (ident := term) in *, using a logical (Leibniz’s) equality instead of a local definition. Use naming_intropattern to name or split up the new equation.

Variant remember term as ident1 eqn:naming_intropattern? in goal_occurrences

This is a more general form of remember that remembers the occurrences of term specified by an occurrence set.

Variant eremember term as ident1 eqn:naming_intropattern? in goal_occurrences?

While the different variants of remember expect that no existential variables are generated by the tactic, eremember removes this constraint.

Tactic pose (ident := term)

This adds the local definition ident := term to the current context without performing any replacement in the goal or in the hypotheses. It is equivalent to set (ident := term) in |-.

Variant pose (ident binder* := term)

This is equivalent to pose (ident := fun binder* => term).

Variant pose term

This behaves as pose (ident := term) but ident is generated by Coq.

Variant epose (ident binder* := term)
Variant epose term

While the different variants of pose expect that no existential variables are generated by the tactic, epose removes this constraint.

Tactic decompose [qualid+] term

This tactic recursively decomposes a complex proposition in order to obtain atomic ones.

Example

Goal forall A B C:Prop, A /\ B /\ C \/ B /\ C \/ C /\ A -> C.
1 goal ============================ forall A B C : Prop, A /\ B /\ C \/ B /\ C \/ C /\ A -> C
intros A B C H; decompose [and or] H.
3 goals A, B, C : Prop H : A /\ B /\ C \/ B /\ C \/ C /\ A H1 : A H0 : B H3 : C ============================ C goal 2 is: C goal 3 is: C
all: assumption.
No more goals.
Qed.

Note

decompose does not work on right-hand sides of implications or products.

Variant decompose sum term

This decomposes sum types (like or).

Variant decompose record term

This decomposes record types (inductive types with one constructor, like and and exists and those defined with the Record command.

## Controlling the proof flow¶

Tactic assert (ident : type)

This tactic applies to any goal. assert (H : U) adds a new hypothesis of name H asserting U to the current goal and opens a new subgoal U 2. The subgoal U comes first in the list of subgoals remaining to prove.

Error Not a proposition or a type.

Arises when the argument type is neither of type Prop, Set nor Type.

Variant assert type

This behaves as assert (ident : type) but ident is generated by Coq.

Variant assert type by tactic

This tactic behaves like assert but applies tactic to solve the subgoals generated by assert.

Error Proof is not complete.
Variant assert type as simple_intropattern

If simple_intropattern is an intro pattern (see Intro patterns), the hypothesis is named after this introduction pattern (in particular, if simple_intropattern is ident, the tactic behaves like assert (ident : type)). If simple_intropattern is an action introduction pattern, the tactic behaves like assert type followed by the action done by this introduction pattern.

Variant assert type as simple_intropattern by tactic

This combines the two previous variants of assert.

Variant assert (ident := term)

This behaves as assert (ident : type) by exact term where type is the type of term. This is equivalent to using pose proof. If the head of term is ident, the tactic behaves as specialize.

Error Variable ident is already declared.
Variant eassert type as simple_intropattern by tactic

While the different variants of assert expect that no existential variables are generated by the tactic, eassert removes this constraint. This lets you avoid specifying the asserted statement completely before starting to prove it.

Variant pose proof term as simple_intropattern?

This tactic behaves like assert type as simple_intropattern? by exact term where type is the type of term. In particular, pose proof term as ident behaves as assert (ident := term) and pose proof term as simple_intropattern is the same as applying the simple_intropattern to term.

Variant epose proof term as simple_intropattern?

While pose proof expects that no existential variables are generated by the tactic, epose proof removes this constraint.

Variant pose proof (ident := term)

This is an alternative syntax for assert (ident := term) and pose proof term as ident, following the model of pose (ident := term) but dropping the value of ident.

Variant epose proof (ident := term)

This is an alternative syntax for eassert (ident := term) and epose proof term as ident, following the model of epose (ident := term) but dropping the value of ident.

Variant enough (ident : type)

This adds a new hypothesis of name ident asserting type to the goal the tactic enough is applied to. A new subgoal stating type is inserted after the initial goal rather than before it as assert would do.

Variant enough type

This behaves like enough (ident : type) with the name ident of the hypothesis generated by Coq.

Variant enough type as simple_intropattern

This behaves like enough type using simple_intropattern to name or destruct the new hypothesis.

Variant enough (ident : type) by tactic
Variant enough type as simple_intropattern? by tactic

This behaves as above but with tactic expected to solve the initial goal after the extra assumption type is added and possibly destructed. If the as simple_intropattern clause generates more than one subgoal, tactic is applied to all of them.

Variant eenough type as simple_intropattern? by tactic?
Variant eenough (ident : type) by tactic?

While the different variants of enough expect that no existential variables are generated by the tactic, eenough removes this constraint.

Variant cut type

This tactic applies to any goal. It implements the non-dependent case of the “App” rule given in Typing rules. (This is Modus Ponens inference rule.) cut U transforms the current goal T into the two following subgoals: U -> T and U. The subgoal U -> T comes first in the list of remaining subgoal to prove.

Variant specialize (ident term*) as simple_intropattern?
Variant specialize ident with bindings as simple_intropattern?

This tactic works on local hypothesis ident. The premises of this hypothesis (either universal quantifications or non-dependent implications) are instantiated by concrete terms coming either from arguments term* or from Bindings. In the first form the application to term* can be partial. The first form is equivalent to assert (ident := ident term*). In the second form, instantiation elements can also be partial. In this case the uninstantiated arguments are inferred by unification if possible or left quantified in the hypothesis otherwise. With the as clause, the local hypothesis ident is left unchanged and instead, the modified hypothesis is introduced as specified by the simple_intropattern. The name ident can also refer to a global lemma or hypothesis. In this case, for compatibility reasons, the behavior of specialize is close to that of generalize: the instantiated statement becomes an additional premise of the goal. The as clause is especially useful in this case to immediately introduce the instantiated statement as a local hypothesis.

Error ident is used in hypothesis ident.
Error ident is used in conclusion.
Tactic generalize term

This tactic applies to any goal. It generalizes the conclusion with respect to some term.

Example

Goal forall x y:nat, 0 <= x + y + y.
1 goal ============================ forall x y : nat, 0 <= x + y + y
Proof. intros *.
1 goal x, y : nat ============================ 0 <= x + y + y
Show.
1 goal x, y : nat ============================ 0 <= x + y + y
generalize (x + y + y).
1 goal x, y : nat ============================ forall n : nat, 0 <= n

If the goal is G and t is a subterm of type T in the goal, then generalize t replaces the goal by forall (x:T), G′ where G′ is obtained from G by replacing all occurrences of t by x. The name of the variable (here n) is chosen based on T.

Variant generalize term+

This is equivalent to generalize term; ... ; generalize term. Note that the sequence of term i 's are processed from n to 1.

Variant generalize term at natural+

This is equivalent to generalize term but it generalizes only over the specified occurrences of term (counting from left to right on the expression printed using the Printing All flag).

Variant generalize term as ident

This is equivalent to generalize term but it uses ident to name the generalized hypothesis.

Variant generalize term at natural+ as ident+,

This is the most general form of generalize that combines the previous behaviors.

Variant generalize dependent term

This generalizes term but also all hypotheses that depend on term. It clears the generalized hypotheses.

Tactic evar (ident : term)

The evar tactic creates a new local definition named ident with type term in the context. The body of this binding is a fresh existential variable.

Tactic instantiate (ident := term )

The instantiate tactic refines (see refine) an existential variable ident with the term term. It is equivalent to only [ident]: refine term (preferred alternative).

Note

To be able to refer to an existential variable by name, the user must have given the name explicitly (see Existential variables).

Note

When you are referring to hypotheses which you did not name explicitly, be aware that Coq may make a different decision on how to name the variable in the current goal and in the context of the existential variable. This can lead to surprising behaviors.

Variant instantiate (natural := term)

This variant selects an existential variable by its position. The natural argument is the position of the existential variable from right to left in the conclusion of the goal. (Use one of the variants below to select an existential variable in a hypothesis.) Counting starts at 1 and multiple occurrences of the same existential variable are counted multiple times. Because this variant is not robust to slight changes in the goal, its use is strongly discouraged.

Variant instantiate ( natural := term ) in ident
Variant instantiate ( natural := term ) in ( value of ident )
Variant instantiate ( natural := term ) in ( type of ident )

These allow to refer respectively to existential variables occurring in a hypothesis or in the body or the type of a local definition (named ident).

Variant instantiate

Without argument, the instantiate tactic tries to solve as many existential variables as possible, using information gathered from other tactics in the same tactical. This is automatically done after each complete tactic (i.e. after a dot in proof mode), but not, for example, between each tactic when they are sequenced by semicolons.

Tactic admit

This tactic allows temporarily skipping a subgoal so as to progress further in the rest of the proof. A proof containing admitted goals cannot be closed with Qed but only with Admitted.

Variant give_up

Synonym of admit.

Tactic absurd term

This tactic applies to any goal. The argument term is any proposition P of type Prop. This tactic applies False elimination, that is it deduces the current goal from False, and generates as subgoals ∼P and P. It is very useful in proofs by cases, where some cases are impossible. In most cases, P or ∼P is one of the hypotheses of the local context.

Tactic contradiction

This tactic applies to any goal. The contradiction tactic attempts to find in the current context (after all intros) a hypothesis that is equivalent to an empty inductive type (e.g. False), to the negation of a singleton inductive type (e.g. True or x=x), or two contradictory hypotheses.

Error No such assumption.
Variant contradiction ident

The proof of False is searched in the hypothesis named ident.

Tactic contradict ident

This tactic allows manipulating negated hypothesis and goals. The name ident should correspond to a hypothesis. With contradict H, the current goal and context is transformed in the following way:

• H:¬A ⊢ B becomes ⊢ A

• H:¬A ⊢ ¬B becomes H: B ⊢ A

• H: A ⊢ B becomes ⊢ ¬A

• H: A ⊢ ¬B becomes H: B ⊢ ¬A

Tactic exfalso

This tactic implements the “ex falso quodlibet” logical principle: an elimination of False is performed on the current goal, and the user is then required to prove that False is indeed provable in the current context. This tactic is a macro for elimtype False.

## Checking properties of terms¶

Each of the following tactics acts as the identity if the check succeeds, and results in an error otherwise.

Tactic constr_eq term term

This tactic checks whether its arguments are equal modulo alpha conversion, casts and universe constraints. It may unify universes.

Error Not equal.
Error Not equal (due to universes).
Tactic constr_eq_strict term term

This tactic checks whether its arguments are equal modulo alpha conversion, casts and universe constraints. It does not add new constraints.

Error Not equal.
Error Not equal (due to universes).
Tactic unify term term

This tactic checks whether its arguments are unifiable, potentially instantiating existential variables.

Error Unable to unify term with term.
Variant unify term term with ident

Unification takes the transparency information defined in the hint database ident into account (see the hints databases for auto and eauto).

Tactic is_evar term

This tactic checks whether its argument is a current existential variable. Existential variables are uninstantiated variables generated by eapply and some other tactics.

Error Not an evar.
Tactic has_evar term

This tactic checks whether its argument has an existential variable as a subterm. Unlike context patterns combined with is_evar, this tactic scans all subterms, including those under binders.

Error No evars.
Tactic is_var term

This tactic checks whether its argument is a variable or hypothesis in the current local context.

Error Not a variable or hypothesis.

## Classical tactics¶

In order to ease the proving process, when the Classical module is loaded, a few more tactics are available. Make sure to load the module using the Require Import command.

Tactic classical_left
Tactic classical_right

These tactics are the analog of left and right but using classical logic. They can only be used for disjunctions. Use classical_left to prove the left part of the disjunction with the assumption that the negation of right part holds. Use classical_right to prove the right part of the disjunction with the assumption that the negation of left part holds.

## Delaying solving unification constraints¶

Tactic solve_constraints
Flag Solve Unification Constraints

By default, after each tactic application, postponed typechecking unification problems are resolved using heuristics. Unsetting this flag disables this behavior, allowing tactics to leave unification constraints unsolved. Use the solve_constraints tactic at any point to solve the constraints.

## Proof maintenance¶

Experimental. Many tactics, such as intros, can automatically generate names, such as "H0" or "H1" for a new hypothesis introduced from a goal. Subsequent proof steps may explicitly refer to these names. However, future versions of Coq may not assign names exactly the same way, which could cause the proof to fail because the new names don't match the explicit references in the proof.

The following "Mangle Names" settings let users find all the places where proofs rely on automatically generated names, which can then be named explicitly to avoid any incompatibility. These settings cause Coq to generate different names, producing errors for references to automatically generated names.

Flag Mangle Names

When this flag is set (it is off by default), generated names use the prefix specified in the following option instead of the default prefix.

Option Mangle Names Prefix string

This option specifies the prefix to use when generating names.

## Performance-oriented tactic variants¶

Tactic exact_no_check term

For advanced usage. Similar to exact term, but as an optimization, it skips checking that term has the goal's type, relying on the kernel check instead. See change_no_check for more explanation.

Example

Goal False.
1 goal ============================ False
exact_no_check I.
No more goals.
Fail Qed.
The command has indeed failed with message: The term "I" has type "True" while it is expected to have type "False".
Variant vm_cast_no_check term

For advanced usage. Similar to exact_no_check term, but additionally instructs the kernel to use vm_compute to compare the goal's type with the term's type.

Example

Goal False.
1 goal ============================ False
vm_cast_no_check I.
No more goals.
Fail Qed.
The command has indeed failed with message: The term "I" has type "True" while it is expected to have type "False".
Variant native_cast_no_check term

for advanced usage. similar to exact_no_check term, but additionally instructs the kernel to use native_compute to compare the goal's type with the term's type.

Example

Goal False.
1 goal ============================ False
native_cast_no_check I.
No more goals.
Fail Qed.
The command has indeed failed with message: Native compiler is disabled, falling back to VM conversion test. [native-compiler-disabled,native-compiler]
1

Actually, only the second subgoal will be generated since the other one can be automatically checked.

2

This corresponds to the cut rule of sequent calculus.