The critical new ideas are polymorphism (abstracting functions over the types of the data they manipulate) and higher-order functions (treating functions as data).
Polymorphism
Until today, We were living in the monomorphic world of Coq. So if we want a list, we have to define it for each type:
But of course Coq supports polymorphic type. So we can abstract things over type
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Inductive list (X:Type) : Type := | nil | cons (x : X) (l : list X).
Check list. (* ===> list : Type -> Type *)
Recall from PLT course, this is exacly Parametric Polymorphism and it’s SystemFω. the list here is a type-level small lambda, or type operators
Another things I’d love to mention is the concrete syntax of list X, it didn’t choose the SML/OCaml order but the Haskell order.
Q1. What’s the type of nil and cons?
Both having forall type
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Check nil. (* ===> nil : forall X : Type, list X *) Check cons. (* ===> nil : forall X : Type, X -> list X -> list X *)
Q2. What’s the type of list nat? Why not Type but weird Set?
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Check nat. (* ===> nat : Set *) Check list nat. (* ===> list nat : Set *) CheckSet. (* ===> Set: Type *) CheckType. (* ===> Type: Type *)
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Check (cons nat 2 (cons nat 1 (nil nat))).
Polymorphic Functions
we can make polymorphic versions of list-processing function:
Btw, Pierce follows the TAPL convention where type is written in capital letter but not greek letter, less clear in first look but better for typing in real programming.
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Fixpointrepeat (X : Type) (x : X) (count : nat) : list X := match count with | 0 ⇒ nil X | S count' ⇒ cons X x (repeat X x count') end.
This is SystemF.
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Check repeat. (* ===> repeat : forall X : Type, X -> nat -> list X *)
Slide QA
ill-typed
forall X : Type, X -> nat -> list X
list nat
Type Argument Inference
X must be a Type since nil expects an Type as its first argument.
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Fixpointrepeat' X x count : list X := (* return type [:list X] can be omitted as well *) match count with | 0 ⇒ nil X | S count' ⇒ cons X x (repeat' X x count') end.
Checkrepeat'. (* ===> forall X : Type, X -> nat -> list X *)
Type Argument Synthesis
We can write _ (hole) in place of X and Coq will try to unify all local information.
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Fixpointrepeat'' X x count : list X := match count with | 0 ⇒ nil _ | S count' ⇒ cons _ x (repeat'' _ x count') end.
repeat' X x count : list X := repeat' (X : _) (x : _) (count : _) : list X :=
Implicit Arguments
Using Arguments directives to tell if an argument need to be implicit (i.e. omitted and always to infer) or not.
Implicitly convert to _ (synthesis) by frontend.
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Arguments nil {X}. Arguments cons {X} __. (* data constructor usually don't specify the name *) Argumentsrepeat {X} x count. (* fun definition usually do *)
The even more convenient syntax is that we can declare them right in our function definition. Just surrounding them with curly braces.
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Fixpointrepeat''' {X : Type} (x : X) (count : nat) : list X := match count with | 0 ⇒ nil | S count' ⇒ cons x (repeat''' x count') end.
Doing this will make X implicit for even list', the type constructor itself…
Other Polymorphic List functions
No difference but add implicit type argument {X : Type}.
Supplying Type Arguments Explicitly
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Fail Definition mynil := nil.
Definition mynil : list nat := nil.
Check @nil. (* ===> @nil : forall X : Type, list X *) Definition mynil' := @nil nat.
First thought: Existential Second thought: A wait to be unified Universal. (after being implicit and require inference)
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Check nil.
nil : list ?X where ?X : [ |- Type]
List notation
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Notation"x :: y" := (cons x y) (at level 60, right associativity). Notation"[ ]" := nil. Notation"[ x ; .. ; y ]" := (cons x .. (cons y []) ..). Notation"x ++ y" := (app x y) (at level 60, right associativity).
Same with before thanks to the implicit argument
Slide Q&A 2
we use ; not ,!!
list nat
ill-typed
ill-typed
list (list nat)
list (list nat) (tricky in first look)
list bool
ill-typed
ill-typed
Poly Pair
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Inductive prod (X Y : Type) : Type := | pair (x : X) (y : Y). Arguments pair {X} {Y} __. (* omit two type var **)
Notation"( x , y )" := (pair x y). Notation"X * Y" := (prod X Y) : type_scope. (* only be used when parsing type, avoids clashing with multiplication *)
Be careful of (X,Y) and X*Y. Coq pick the ML way, not haskell way.
Combine or Zip
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Fixpoint combine {X Y : Type} (lx : list X) (ly : list Y) : list (X*Y) := match lx, ly with | [], _ ⇒ [] | _, [] ⇒ [] | x :: tx, y :: ty ⇒ (x, y) :: (combine tx ty) end.
Guess type?
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Check @combine. @combine : forall X Y : Type, list X -> list Y -> list (X * Y)
(* A special form of `forall`? *) Check combine. combine : list ?X -> list ?Y -> list (?X * ?Y) where ?X : [ |- Type] ?Y : [ |- Type]
Poly Option
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Inductive option (X:Type) : Type := | Some (x : X) | None.
Arguments Some {X} _. Arguments None {X}.
(* find nth element if exist, None otherwise *) Fixpoint nth_error {X : Type} (l : list X) (n : nat) : option X := match l with | [] ⇒ None | a :: l' ⇒ if n =? O then Some a else nth_error l' (pred n) end.
Function as data
Functions as first-class citizens
Higher-Order Functions
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Definition doit3times {X:Type} (f:X→X) (n:X) : X := f (f (f n)).
Check @doit3times. (* ===> doit3times : forall X : Type, (X -> X) -> X -> X *)
Filter (taking a predicate on X)
collection-oriented programming style - my first time seeing this, any comments?
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Fixpoint filter {X:Type} (test: X→bool) (l:list X) : (list X) := match l with | [] ⇒ [] | h :: t ⇒ if test h then h :: (filter test t) else filter test t end.
Example test_filter1: filter evenb [1;2;3;4] = [2;4]. Proof. reflexivity. Qed.
Anonymous Functions
It is arguably a little sad, in the example just above, to be forced to define the function length_is_1 and give it a name just to be able to pass it as an argument to filter
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Example test_anon_fun': doit3times (fun n ⇒ n * n) 2 = 256. Proof. reflexivity. Qed.
Syntax: hybrid of OCaml fun n -> n and SML fn n => n. and support multi-arguments (curried)
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Compute ((fun x y => x + y) 35).
Map
Should be familar.
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Fixpoint map {X Y: Type} (f:X→Y) (l:list X) : (list Y) := match l with | [] ⇒ [] | h :: t ⇒ (f h) :: (map f t) end.
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Check @map
@map : forall X Y : Type, (X -> Y) -> list X -> list Y
Slide Q&A 3
as above
option map
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Definition option_map {X Y : Type} (f : X → Y) (xo : option X) : option Y := match xo with | None ⇒ None | Some x ⇒ Some (f x) end.
Functor Map (fmap) !
Fold (Reduce)
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Fixpointfold {X Y: Type} (f: X→Y→Y) (l: list X) (b: Y) : Y := match l with | nil ⇒ b | h :: t ⇒ f h (fold f t b) end.
Fold Right (foldr). Argument order same with OCaml, different with Haskell.
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Check @fold
@fold : forall X Y : Type, (X -> Y -> Y) -> list X -> Y -> Y
I encounter this problem when doing church numeral exercise.
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Definition plus (n m : cnat) : cnat := n cnat succ m.
will result in universe inconsistency error.
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SetPrintingUniverses. (* giving more error msg *)
In environment n : cnat m : cnat The term "cnat" has type "Type@{Top.168+1}" while it is expected to have type "Type@{Top.168}" (universe inconsistency: Cannot enforce Top.168 < Top.168 because Top.168 = Top.168).
What’s happening?
Yes, you can define: Definition plus (n m : cnat) : cnat := n cnat succ m. in System F. However, in Coq’s richer logic, you need to be a little more careful about allowing types to be instantiated at their own types, else you run into issue of inconsistency. Essentially, there is a stratification of types (by “universes”) that says that one universe cannot contain a “bigger” universe. Often, things are polymorphic in their universe (i.e., work in all universes), you run into this where you cannot instantiate the “forall X, …” that is the definition of cnat by cnat itself. — Prof. Fluet
Thus, the correct answer for that question is that Type_i has type Type_j, for any index j > i. This is needed to ensure the consistency of Coq’s theory: if there were only one Type, it would be possible to show a contradiction, similarly to how one gets a contradiction in set theory if you assume that there is a set of all sets. Coq generates one new index variable every time you write Type, and keeps track of internal constraints
The error message you saw means that Coq’s constraint solver for universe levels says that there can’t be a solution to the constraint system you asked for.
The problem is that the forall in the definition of nat is quantified over Type_i, but Coq’s logic forces nat to be itself of type Type_j, with j > i. On the other hand, the application n nat requires that j <= i, resulting in a non-satisfiable set of index constraints.
The command has indeed failed with message: The term "@identity" has type "forall A : Type, A -> A" while it is expected to have type "?A" (unable to find a well-typed instantiation for"?A": cannot ensure that "Type@{Top.1+1}" is a subtype of "Type@{Top.1}").
The link also introduce some advanced/experimental way to do polymorphic universe
Polymorphic Church Numerals w/o self-applying itself