Implicit Coercions¶
 Author
Amokrane Saïbi
General Presentation¶
This section describes the inheritance mechanism of Coq. In Coq with inheritance, we are not interested in adding any expressive power to our theory, but only convenience. Given a term, possibly not typable, we are interested in the problem of determining if it can be well typed modulo insertion of appropriate coercions. We allow to write:
f a
wheref:(forall x:A,B)
anda:A'
whenA'
can be seen in some sense as a subtype ofA
.
x:A
whenA
is not a type, but can be seen in a certain sense as a type: set, group, category etc.
f a
whenf
is not a function, but can be seen in a certain sense as a function: bijection, functor, any structure morphism etc.
Coercion Classes¶
A class with \(n\) parameters is any defined name with a type
forall (ident_{1} : type_{1})..(ident_{n}:type_{n}), sort
. Thus a class with
parameters is considered as a single class and not as a family of
classes. An object of a coercion class is any term of type
coercion_class term_{1} .. term_{n}
.
In addition to these userdefined classes, we have two builtin classes:
Sortclass
, the class of sorts; its objects are the terms whose type is a sort (e.g.Prop
orType
).
Funclass
, the class of functions; its objects are all the terms with a functional type, i.e. of formforall x:A,B
.
Formally, the syntax of classes is defined as:
coercion_class::=
Funclass
Sortclass
reference
Note
Don't confuse coercion classes with typeclasses, which are records with
special properties defined with the Class
command.
Coercions¶
A name f
can be declared as a coercion between a source userdefined class
C
with \(n\) parameters and a target class D
if one of these
conditions holds:
D
is a userdefined class, then the type off
must have the formforall (x₁:A₁)..(xₖ:Aₖ)(y:C v₁..vₙ), D u₁..uₘ
where \(m\) is the number of parameters ofD
.
D
isFunclass
, then the type off
must have the formforall (x₁:A₁)..(xₖ:Aₖ)(y:C v₁..vₙ)(x:A), B
.
D
isSortclass
, then the type off
must have the formforall (x₁:A₁)..(xₖ:Aₖ)(y:C v₁..vₙ), s
withs
a sort.
We then write f : C >> D
.
When you declare a new coercion (e.g. with Coercion
), new coercion
paths with the same classes as existing ones are ignored. Coq will generate
a warning when the two paths may be non convertible. When the x₁..xₖ
are exactly
the v₁..vₙ
(in the same order), the coercion is said to satisfy
the uniform inheritance condition. When possible, we recommend
using coercions that satisfy this condition. This guarantees that
no spurious warning will be generated.
Note
The builtin class Sortclass
can be used as a source class, but
the builtin class Funclass
cannot.
To coerce an object t:C t₁..tₙ
of C
towards D
, we have to
apply the coercion f
to it; the obtained term f _.._ t
is
then an object of D
.
Reversible Coercions¶
When a term cannot be coerced (directly) to its expected type, Coq tries to
use a reversible coercion (see the reversible
attribute). Intuitively,
Coq synthesizes a new term of the right type that can be coerced
to the original one. The new term is obtained by reversing the coercion, that
is guessing its input given the output.
More precisely, in order to coerce a term a : A
to type B
, Coq
finds a reversible coercion f : B >> A
, then synthesizes some ?x : B
such that f ?x = a
(typically through Canonical Structures or
Typeclasses) and finally replaces a
with the value of ?x
.
If Coq doesn't find a reversible coercion f : B >> A
, then it
looks for a coercion class C
equipped with an incoming reversible coercion
g : B >> C
and a coercion h : A >> C
(not necessarily reversible),
then synthesizes some ?x : B
such that g ?x = h a
, and finally
replaces a
with the value of ?x
.
If there's another class D
with a coercion from C
to D
and
incoming coercions from A
and B
, Coq tries C
before D
.
This ordering is well defined only if the coercion graph happens to be a semi
lattice. The intuition behind this ordering is that since coercions forget
information, D
has less information that C
, and hence
inferring ?x : B
from h a : D
would be harder.
See the example below.
Identity Coercions¶
To make coercions work for both a named class and for
Sortclass
or Funclass
, use the Identity Coercion
command.
There is an example here.
Inheritance Graph¶
Coercions form an inheritance graph with classes as nodes. We call
coercion path an ordered list of coercions between two nodes of
the graph. A class C
is said to be a subclass of D
if there is a
coercion path in the graph from C
to D
; we also say that C
inherits from D
. Our mechanism supports multiple inheritance since a
class may inherit from several classes, contrary to simple inheritance
where a class inherits from at most one class. However there must be
at most one path between two classes. If this is not the case, only
the oldest one is valid and the others are ignored. So the order
of declaration of coercions is important.
We extend notations for coercions to coercion paths. For instance
[f₁;..;fₖ] : C >> D
is the coercion path composed
by the coercions f₁..fₖ
. The application of a coercion path to a
term consists of the successive application of its coercions.
Coercion Classes¶

Command
Coercion reference : coercion_class >> coercion_class?
¶ 
Command
Coercion ident_decl def_body
The first form declares the construction denoted by
reference
as a coercion between the two given classes. The second form definesident_decl
just likeDefinition
ident_decl def_body
and then declaresident_decl
as a coercion between it source and its target. Both forms support thelocal
attribute, which makes the coercion local to the current section.: coercion_class >> coercion_class?
The source and target classes of the coercion. If unspecified,
reference
must already be a coercion, which enables modifying thereversible
attribute ofreference
. See the example below.

Attribute
reversible= yesno?
¶ This attribute allows the coercion to be used as a reversible coercion. By default coercions are not reversible except for
Record
fields specified using:>
.

Attribute
nonuniform
¶ Silence the non uniform inheritance warning.

Error
Funclass cannot be a source class.
¶ Funclass as a source class is currently not supported. This may change in the future.

Error
Cannot recognize coercion_class as a source class of qualid.
¶ The inferred source class of the coercion differs from the one specified.

Error
Cannot find the target class
¶ The target class of the coercion is not specified and cannot be inferred. Make sure that the target is not a variable.

Error
Found target class coercion_class instead of coercion_class
¶ The inferred target class of the coercion differs from the one specified.

Warning
qualid does not respect the uniform inheritance condition.
¶ The test for ambiguous coercion paths may yield false positives involving the coercion
qualid
. Use thenonuniform
attribute to silence this warning.

Warning
New coercion path ... is ambiguous with existing ...
¶ The check for ambiguous paths failed. The paths for which this check fails are displayed by a warning in the form
[f₁;..;fₙ] : C >> D
.The convertibility checking procedure for coercion paths is complete for paths consisting of coercions satisfying the uniform inheritance condition, but some coercion paths could be reported as ambiguous even if they are convertible with existing ones when they have coercions that don't satisfy this condition.

Warning
... is not definitionally an identity function.
¶ If a coercion path has the same source and target class, that is said to be circular. When a new circular coercion path is not convertible with the identity function, it will be reported as ambiguous.
Some objects can be declared as coercions when they are defined.
This applies to assumptions and
constructors of inductive types and record fields.
Use :>
instead of :
before the
type of the assumption to do so. See of_type
.

Command
Identity Coercion ident : coercion_class_{src} >> coercion_class_{dest}
¶ Checks that
coercion_class_{src}
is a constant with a body of the formfun (x₁:T₁)..(xₙ:Tₙ) => coercion_class_{dest} t₁..tₘ
wherem
is the number of parameters ofcoercion_class_{dest}
. Then we define an identity function with typeforall (x₁:T₁)..(xₙ:Tₙ)(y:C x₁..xₙ),D t₁..tₘ
, and we declare it as an identity coercion betweenC
andD
. See below for an example.This command supports the
local
attribute, which makes the coercion local to the current section.
Error
coercion_class must be a transparent constant.
¶

Command
SubClass ident_decl def_body
¶ If
type
is a coercion classident'
applied to some arguments thenident
is defined and an identity coercion of nameId_ident_ident'
is declared. In other words, this is an abbreviation forDefinition ident := type.
Identity Coercion Id_ident_ident' : ident >> ident'
.This command supports the
local
attribute, which makes the coercion local to the current section.

Error
Displaying Available Coercions¶

Command
Print Classes
¶ Print the list of declared coercion classes in the current context.

Command
Print Coercions
¶ Print the list of declared coercions in the current context.

Command
Print Graph
¶ Print the list of valid coercion paths in the current context.

Command
Print Coercion Paths coercion_class coercion_class
¶ Print the list of valid coercion paths between the two given classes.
Activating the Printing of Coercions¶
Classes as Records¶
Structures with Inheritance may be defined using the Record
command.
Use >
before the record name to declare the constructor name as
a coercion from the class of the last field type to the record name.
See record_definition
.
Use :>
in the field type to declare the field as a coercion from the
record name to the class of the field type. For these coercions, the
reversible
attribute defaults to yes
. See of_type
.
Coercions and Sections¶
The inheritance mechanism is compatible with the section mechanism. The global classes and coercions defined inside a section are redefined after its closing, using their new value and new type. The classes and coercions which are local to the section are simply forgotten. Coercions with a local source class or a local target class are also forgotten.
Coercions and Modules¶
The coercions present in a module are activated only when the module is explicitly imported.
Examples¶
There are three situations:
Example: Coercion at function application
f a
is illtyped where f:forall x:A,B
and a:A'
. If there is a
coercion path between A'
and A
, then f a
is transformed into
f a'
where a'
is the result of the application of this
coercion path to a
.
We first give an example of coercion between atomic inductive types
 Definition bool_in_nat (b:bool) := if b then 0 else 1.
 bool_in_nat is defined
 Coercion bool_in_nat : bool >> nat.
 bool_in_nat is now a coercion
 Check (0 = true).
 0 = true : Prop
 Set Printing Coercions.
 Check (0 = true).
 0 = bool_in_nat true : Prop
 Unset Printing Coercions.
Warning
Note that Check (true = O)
would fail. This is "normal" behavior of
coercions. To validate true=O
, the coercion is searched from
nat
to bool
. There is none.
We give an example of coercion between classes with parameters.
 Parameters (C : nat > Set) (D : nat > bool > Set) (E : bool > Set).
 C is declared D is declared E is declared
 Parameter f : forall n:nat, C n > D (S n) true.
 f is declared
 Coercion f : C >> D.
 f is now a coercion
 Parameter g : forall (n:nat) (b:bool), D n b > E b.
 g is declared
 Coercion g : D >> E.
 g is now a coercion
 Parameter c : C 0.
 c is declared
 Parameter T : E true > nat.
 T is declared
 Check (T c).
 T c : nat
 Set Printing Coercions.
 Check (T c).
 T (g 1 true (f 0 c)) : nat
 Unset Printing Coercions.
In the case of functional arguments, we use the monotonic rule of
subtyping. To coerce t : forall x : A, B
towards
forall x : A', B'
, we have to coerce A'
towards A
and B
towards B'
. An example is given below:
 Parameters (A B : Set) (h : A > B).
 A is declared B is declared h is declared
 Coercion h : A >> B.
 h is now a coercion
 Parameter U : (A > E true) > nat.
 U is declared
 Parameter t : B > C 0.
 t is declared
 Check (U t).
 U (fun x : A => t x) : nat
 Set Printing Coercions.
 Check (U t).
 U (fun x : A => g 1 true (f 0 (t (h x)))) : nat
 Unset Printing Coercions.
Remark the changes in the result following the modification of the previous example.
 Parameter U' : (C 0 > B) > nat.
 U' is declared
 Parameter t' : E true > A.
 t' is declared
 Check (U' t').
 U' (fun x : C 0 => t' x) : nat
 Set Printing Coercions.
 Check (U' t').
 U' (fun x : C 0 => h (t' (g 1 true (f 0 x)))) : nat
 Unset Printing Coercions.
Example: Coercion to a type
An assumption x:A
when A
is not a type, is illtyped. It is
replaced by x:A'
where A'
is the result of the application to
A
of the coercion path between the class of A
and
Sortclass
if it exists. This case occurs in the abstraction
fun x:A => t
, universal quantification forall x:A,B
, global
variables and parameters of (co)inductive definitions and
functions. In forall x:A,B
, such a coercion path may also be applied
to B
if necessary.
 Parameter Graph : Type.
 Graph is declared
 Parameter Node : Graph > Type.
 Node is declared
 Coercion Node : Graph >> Sortclass.
 Node is now a coercion
 Parameter G : Graph.
 G is declared
 Parameter Arrows : G > G > Type.
 Arrows is declared
 Check Arrows.
 Arrows : G > G > Type
 Parameter fg : G > G.
 fg is declared
 Check fg.
 fg : G > G
 Set Printing Coercions.
 Check fg.
 fg : Node G > Node G
 Unset Printing Coercions.
Example: Coercion to a function
f a
is illtyped because f:A
is not a function. The term
f
is replaced by the term obtained by applying to f
the
coercion path between A
and Funclass
if it exists.
 Parameter bij : Set > Set > Set.
 bij is declared
 Parameter ap : forall A B:Set, bij A B > A > B.
 ap is declared
 Coercion ap : bij >> Funclass.
 ap is now a coercion
 Parameter b : bij nat nat.
 b is declared
 Check (b 0).
 b 0 : nat
 Set Printing Coercions.
 Check (b 0).
 ap nat nat b 0 : nat
 Unset Printing Coercions.
Example: Reversible coercions
Notice the :>
on ssort
making it a reversible coercion.
 Structure S := { ssort :> Type; sstuff : ssort; }.
 S is defined ssort is defined sstuff is defined
 Definition test (s : S) := sstuff s.
 test is defined
 Canonical Structure S_nat := { ssort := nat; sstuff := 0; }.
 S_nat is defined
 Check test (nat : Type).
 test S_nat : S_nat
Example: Reversible coercions using the reversible
attribute
Notice there is no :>
on ssort'
and the added Coercion
compared
to the previous example.
 Structure S' := { ssort' : Type; sstuff' : ssort'; }.
 S' is defined ssort' is defined sstuff' is defined
 Coercion ssort' : S' >> Sortclass.
 ssort' is now a coercion
 Definition test' (s : S') := sstuff' s.
 test' is defined
 Canonical Structure S_nat' := { ssort' := nat; sstuff' := 0; }.
 S_nat' is defined
Since there's no :>
on the definition of ssort'
, the reversible
attribute is not set:
 Fail Check test' (nat : Type).
 The command has indeed failed with message: The term "nat : Type" has type "Type" while it is expected to have type "S'".
The attribute can be set after declaring the coercion:
 #[reversible] Coercion ssort'.
 Check test' (nat : Type).
 test' S_nat' : S_nat'
Example: Identity coercions.
 Definition fct := nat > nat.
 fct is defined
 Parameter incr_fct : Set.
 incr_fct is declared
 Parameter fct_of_incr_fct : incr_fct > fct.
 fct_of_incr_fct is declared
 Fail Coercion fct_of_incr_fct : incr_fct >> Funclass.
 The command has indeed failed with message: Found target class Funclass instead of fct.
 Coercion fct_of_incr_fct : incr_fct >> fct.
 fct_of_incr_fct is now a coercion
 Parameter f' : incr_fct.
 f' is declared
 Check f' : fct.
 f' : fct : fct
 Fail Check f' 0.
 The command has indeed failed with message: Illegal application (Nonfunctional construction): The expression "f'" of type "incr_fct" cannot be applied to the term "0" : "nat"
 Identity Coercion Id_fct_Funclass : fct >> Funclass.
 Check f' 0.
 f' 0 : nat
Example: Inheritance Graph
Let us see the resulting graph after all these examples.
 Print Graph.
 [h] : A >> B [f] : C >> D [f; g] : C >> E [g] : D >> E [Node] : Graph >> Sortclass [ap] : bij >> Funclass [Id_fct_Funclass] : fct >> Funclass (reversible) [fct_of_incr_fct; Id_fct_Funclass] : incr_fct >> Funclass [fct_of_incr_fct] : incr_fct >> fct [ssort] : S >> Sortclass (reversible) [ssort'] : S' >> Sortclass (reversible) [bool_in_nat] : bool >> nat