
# Proof handling¶

In Coq’s proof editing mode all top-level commands documented in Chapter Vernacular commands remain available and the user has access to specialized commands dealing with proof development pragmas documented in this section. They can also use some other specialized commands called tactics. They are the very tools allowing the user to deal with logical reasoning. They are documented in Chapter Tactics.

Coq user interfaces usually have a way of marking whether the user has switched to proof editing mode. For instance, in coqtop the prompt Coq <   is changed into    where ident is the declared name of the theorem currently edited.

At each stage of a proof development, one has a list of goals to prove. Initially, the list consists only in the theorem itself. After having applied some tactics, the list of goals contains the subgoals generated by the tactics.

To each subgoal is associated a number of hypotheses called the local context of the goal. Initially, the local context contains the local variables and hypotheses of the current section (see Section Assumptions) and the local variables and hypotheses of the theorem statement. It is enriched by the use of certain tactics (see e.g. intro).

When a proof is completed, the message Proof completed is displayed. One can then register this proof as a defined constant in the environment. Because there exists a correspondence between proofs and terms of λ-calculus, known as the Curry-Howard isomorphism [How80][Bar81][GLT89][Hue89], Coq stores proofs as terms of CIC. Those terms are called proof terms.

Error No focused proof.

Coq raises this error message when one attempts to use a proof editing command out of the proof editing mode.

## Entering and leaving proof editing mode¶

The proof editing mode is entered by asserting a statement, which typically is the assertion of a theorem using an assertion command like Theorem. The list of assertion commands is given in Assertions and proofs. The command Goal can also be used.

Command Goal type

This is intended for quick assertion of statements, without knowing in advance which name to give to the assertion, typically for quick testing of the provability of a statement. If the proof of the statement is eventually completed and validated, the statement is then bound to the name Unnamed_thm (or a variant of this name not already used for another statement).

Command Qed

This command is available in interactive editing proof mode when the proof is completed. Then Qed extracts a proof term from the proof script, switches back to Coq top-level and attaches the extracted proof term to the declared name of the original goal. The name is added to the environment as an opaque constant.

Error Attempt to save an incomplete proof.

Note

Sometimes an error occurs when building the proof term, because tactics do not enforce completely the term construction constraints.

The user should also be aware of the fact that since the proof term is completely rechecked at this point, one may have to wait a while when the proof is large. In some exceptional cases one may even incur a memory overflow.

Command Save ident

Saves a completed proof with the name ident, which overrides any name provided by the Theorem command or its variants.

Command Defined

Similar to Qed and Save, except the proof is made transparent, which means that its content can be explicitly used for type checking and that it can be unfolded in conversion tactics (see Performing computations, Opaque, Transparent). If ident is specified, the proof is defined with the given name, which overrides any name provided by the Theorem command or its variants.

This command is available in interactive editing mode to give up the current proof and declare the initial goal as an axiom.

Command Abort Allident?

Cancels the current proof development, switching back to the previous proof development, or to the Coq toplevel if no other proof was being edited.

ident

Aborts editing the proof named ident for use when you have nested proofs. See also Nested Proofs Allowed.

All

Aborts all current proofs.

Error No focused proof (No proof-editing in progress).
Command Proof term

This command applies in proof editing mode. It is equivalent to exact term. Qed. That is, you have to give the full proof in one gulp, as a proof term (see Section Applying theorems).

Warning

Use of this command is discouraged. In particular, it doesn't work in Proof General because it must immediately follow the command that opened proof mode, but Proof General inserts Unset Silent before it (see Proof General issue #498).

Command Proof

Is a no-op which is useful to delimit the sequence of tactic commands which start a proof, after a Theorem command. It is a good practice to use Proof as an opening parenthesis, closed in the script with a closing Qed.

Proof with

Command Proof using section_var_expr with ltac_expr?
|
::=
ident *?
|
Type *?
|
All

Opens proof editing mode, declaring the set of section variables (see Assumptions) used by the proof. At Qed time, the system verifies that the set of section variables used in the proof is a subset of the declared one.

The set of declared variables is closed under type dependency. For example, if T is a variable and a is a variable of type T, then the commands Proof using a and Proof using T a are equivalent.

The set of declared variables always includes the variables used by the statement. In other words Proof using e is equivalent to Proof using Type + e for any declaration expression e.

Use all section variables except those specified by section_var_expr50

Use section variables from the union of both collections. See Name a set of section hypotheses for Proof using to see how to form a named collection.

Use section variables which are in the first collection but not in the second one.

*?

Use the transitive closure of the specified collection.

Type

Use only section variables occurring in the statement. Specifying * uses the forward transitive closure of all the section variables occurring in the statement. For example, if the variable H has type p < 5 then H is in p* since p occurs in the type of H.

All

Use all section variables.

### Proof using options¶

The following options modify the behavior of Proof using.

Option Default Proof Using "section_var_expr"

Use section_var_expr as the default Proof using value. E.g. Set Default Proof Using "a b" will complete all Proof commands not followed by a using part with using a b.

Flag Suggest Proof Using

When Qed is performed, suggest a using annotation if the user did not provide one.

### Name a set of section hypotheses for Proof using¶

Command Collection ident := section_var_expr

This can be used to name a set of section hypotheses, with the purpose of making Proof using annotations more compact.

Example

Define the collection named Some containing x, y and z:

Collection Some := x y z.

Define the collection named Fewer containing only x and y:

Collection Fewer := Some - z

Define the collection named Many containing the set union or set difference of Fewer and Some:

Collection Many := Fewer + Some
Collection Many := Fewer - Some

Define the collection named Many containing the set difference of Fewer and the unnamed collection x y:

Collection Many := Fewer - (x y)
Command Existential num : type? := term

This command instantiates an existential variable. num is an index in the list of uninstantiated existential variables displayed by Show Existentials.

This command is intended to be used to instantiate existential variables when the proof is completed but some uninstantiated existential variables remain. To instantiate existential variables during proof edition, you should use the tactic instantiate.

Command Grab Existential Variables

This command can be run when a proof has no more goal to be solved but has remaining uninstantiated existential variables. It takes every uninstantiated existential variable and turns it into a goal.

### Proof modes¶

When entering proof mode through commands such as Goal and Proof, Coq picks by default the Ltac mode. Nonetheless, there exist other proof modes shipped in the standard Coq installation, and furthermore some plugins define their own proof modes. The default proof mode used when opening a proof can be changed using the following option.

Option Default Proof Mode string

Select the proof mode to use when starting a proof. Depending on the proof mode, various syntactic constructs are allowed when writing an interactive proof. All proof modes support vernacular commands; the proof mode determines which tactic language and set of tactic definitions are available. The possible option values are:

"Classic"

Activates the Ltac language and the tactics with the syntax documented in this manual. Some tactics are not available until the associated plugin is loaded, such as SSR or micromega. This proof mode is set when the prelude is loaded.

"Noedit"

No tactic language is activated at all. This is the default when the prelude is not loaded, e.g. through the -noinit option for coqc.

"Ltac2"

Activates the Ltac2 language and the Ltac2-specific variants of the documented tactics. This value is only available after Requiring Ltac2. Importing Ltac2 sets this mode.

Some external plugins also define their own proof mode, which can be activated with this command.

## Requesting information¶

Command Show ?

Displays the current goals.

num

Display only the num-th subgoal.

ident

Displays the named goal ident. This is useful in particular to display a shelved goal but only works if the corresponding existential variable has been named by the user (see Existential variables) as in the following example.

Example

Goal exists n, n = 0.
1 subgoal ============================ exists n : nat, n = 0
eexists ?[n].
1 focused subgoal (shelved: 1) ============================ ?n = 0
Show n.
subgoal n is: ============================ nat
Error No focused proof.
Error No such goal.
Command Show Proof Diffs removed??

Displays the proof term generated by the tactics that have been applied so far. If the proof is incomplete, the term will contain holes, which correspond to subterms which are still to be constructed. Each hole is an existential variable, which appears as a question mark followed by an identifier.

Experimental: Specifying “Diffs” highlights the difference between the current and previous proof step. By default, the command shows the output once with additions highlighted. Including “removed” shows the output twice: once showing removals and once showing additions. It does not examine the Diffs option. See Showing differences between proof steps.

Command Show Conjectures

Prints the names of all the theorems that are currently being proved. As it is possible to start proving a previous lemma during the proof of a theorem, there may be multiple names.

Command Show Intro

If the current goal begins by at least one product, prints the name of the first product as it would be generated by an anonymous intro. The aim of this command is to ease the writing of more robust scripts. For example, with an appropriate Proof General macro, it is possible to transform any anonymous intro into a qualified one such as intro y13. In the case of a non-product goal, it prints nothing.

Command Show Intros

Similar to the previous command. Simulates the naming process of intros.

Command Show Existentials

Displays all open goals / existential variables in the current proof along with the type and the context of each variable.

Command Show Match qualid

Displays a template of the Gallina match construct with a branch for each constructor of the type qualid. This is used internally by company-coq.

Example

Show Match nat.
match # with | O => | S x => end
Error Unknown inductive type.
Command Show Universes

Displays the set of all universe constraints and its normalized form at the current stage of the proof, useful for debugging universe inconsistencies.

Command Show Goal num at num

Available in coqtop. Displays a goal at a proof state using the goal ID number and the proof state ID number. It is primarily for use by tools such as Prooftree that need to fetch goal history in this way. Prooftree is a tool for visualizing a proof as a tree that runs in Proof General.

Command Guarded

Some tactics (e.g. refine) allow to build proofs using fixpoint or co-fixpoint constructions. Due to the incremental nature of interactive proof construction, the check of the termination (or guardedness) of the recursive calls in the fixpoint or cofixpoint constructions is postponed to the time of the completion of the proof.

The command Guarded allows checking if the guard condition for fixpoint and cofixpoint is violated at some time of the construction of the proof without having to wait the completion of the proof.

## Showing differences between proof steps¶

Coq can automatically highlight the differences between successive proof steps and between values in some error messages. Also, as an experimental feature, Coq can also highlight differences between proof steps shown in the Show Proof command, but only, for now, when using coqtop and Proof General.

For example, the following screenshots of CoqIDE and coqtop show the application of the same intros tactic. The tactic creates two new hypotheses, highlighted in green. The conclusion is entirely in pale green because although it’s changed, no tokens were added to it. The second screenshot uses the "removed" option, so it shows the conclusion a second time with the old text, with deletions marked in red. Also, since the hypotheses are new, no line of old text is shown for them.

This image shows an error message with diff highlighting in CoqIDE:

### How to enable diffs¶

Option Diffs "on""off""removed"

The “on” setting highlights added tokens in green, while the “removed” setting additionally reprints items with removed tokens in red. Unchanged tokens in modified items are shown with pale green or red. Diffs in error messages use red and green for the compared values; they appear regardless of the setting. (Colors are user-configurable.)

For coqtop, showing diffs can be enabled when starting coqtop with the -diffs on|off|removed command-line option or by setting the Diffs option within Coq. You will need to provide the -color on|auto command-line option when you start coqtop in either case.

Colors for coqtop can be configured by setting the COQ_COLORS environment variable. See section By environment variables. Diffs use the tags diff.added, diff.added.bg, diff.removed and diff.removed.bg.

In CoqIDE, diffs should be enabled from the View menu. Don’t use the Set Diffs command in CoqIDE. You can change the background colors shown for diffs from the Edit | Preferences | Tags panel by changing the settings for the diff.added, diff.added.bg, diff.removed and diff.removed.bg tags. This panel also lets you control other attributes of the highlights, such as the foreground color, bold, italic, underline and strikeout.

As of June 2019, Proof General can also display Coq-generated proof diffs automatically. Please see the PG documentation section "Showing Proof Diffs") for details.

### How diffs are calculated¶

Diffs are calculated as follows:

1. Select the old proof state to compare to, which is the proof state before the last tactic that changed the proof. Changes that only affect the view of the proof, such as all: swap 1 2, are ignored.

2. For each goal in the new proof state, determine what old goal to compare it to—the one it is derived from or is the same as. Match the hypotheses by name (order is ignored), handling compacted items specially.

3. For each hypothesis and conclusion (the “items”) in each goal, pass them as strings to the lexer to break them into tokens. Then apply the Myers diff algorithm [Mye86] on the tokens and add appropriate highlighting.

Notes:

• Aside from the highlights, output for the "on" option should be identical to the undiffed output.

• Goals completed in the last proof step will not be shown even with the "removed" setting.

This screen shot shows the result of applying a split tactic that replaces one goal with 2 goals. Notice that the goal P 1 is not highlighted at all after the split because it has not changed.

This is how diffs may appear after applying a intro tactic that results in compacted hypotheses:

## Controlling the effect of proof editing commands¶

Option Hyps Limit num

This option controls the maximum number of hypotheses displayed in goals after the application of a tactic. All the hypotheses remain usable in the proof development. When unset, it goes back to the default mode which is to print all available hypotheses.

Flag Nested Proofs Allowed

When turned on (it is off by default), this flag enables support for nested proofs: a new assertion command can be inserted before the current proof is finished, in which case Coq will temporarily switch to the proof of this nested lemma. When the proof of the nested lemma is finished (with Qed or Defined), its statement will be made available (as if it had been proved before starting the previous proof) and Coq will switch back to the proof of the previous assertion.

## Controlling memory usage¶

Command Print Debug GC

Prints heap usage statistics, which are values from the stat type of the Gc module described here in the OCaml documentation. The live_words, heap_words and top_heap_words values give the basic information. Words are 8 bytes or 4 bytes, respectively, for 64- and 32-bit executables.

When experiencing high memory usage the following commands can be used to force Coq to optimize some of its internal data structures.

Command Optimize Proof

Shrink the data structure used to represent the current proof.

Command Optimize Heap

Perform a heap compaction. This is generally an expensive operation. See: OCaml Gc.compact There is also an analogous tactic optimize_heap.

Memory usage parameters can be set through the OCAMLRUNPARAM environment variable.