### changeset 6:90abb3f53c03

author Yasutaka Higa Thu, 15 Jan 2015 17:27:25 +0900 e8494d175afb c11c259916b7 sandbox/FunctorExample.agda 1 files changed, 60 insertions(+), 0 deletions(-) [+]
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```--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/sandbox/FunctorExample.agda	Thu Jan 15 17:27:25 2015 +0900
@@ -0,0 +1,60 @@
+open import Level
+open import Relation.Binary.PropositionalEquality
+open ≡-Reasoning
+
+
+module FunctorExample where
+
+id : {l : Level} {A : Set l} -> A -> A
+id x = x
+
+_∙_ : {l ll lll : Level} {A : Set l} {B : Set ll} {C : Set lll} -> (B -> C) -> (A -> B) -> (A -> C)
+f ∙ g = \x -> f (g x)
+
+
+
+record Functor {l : Level} (F : Set l -> Set (suc l)) : (Set (suc l)) where
+  field
+    fmap : ∀{A B} -> (A -> B) -> (F A) -> (F B)
+  field
+    preserve-id : ∀{A} (Fa : F A) → fmap id Fa ≡ id Fa
+    covariant : ∀{A B C} (f : A → B) → (g : B → C) → (x : F A)
+      → fmap (g ∙ f) x ≡ fmap g (fmap f x)
+
+data List {l : Level} (A : Set l) : (Set (suc l)) where
+  nil  : List A
+  cons : A -> List A -> List A
+
+list-fmap : {l ll : Level} {A : Set l} {B : Set ll} -> (A -> B) -> List A -> List B
+list-fmap f nil         = nil
+list-fmap f (cons x xs) = cons (f x) (list-fmap f xs)
+
+list-preserve-id : {l : Level} {A : Set l} -> (xs : List A) -> list-fmap id xs ≡ id xs
+list-preserve-id nil = refl
+list-preserve-id (cons x xs) = cong (\li -> cons x li) (list-preserve-id xs)
+
+list-covariant : {l ll lll : Level} {A : Set l} {B : Set ll} {C : Set lll} ->
+                 (f : A -> B) → (g : B -> C) → (x : List A) → list-fmap (g ∙ f) x ≡ list-fmap g (list-fmap f x)
+list-covariant f g nil         = refl
+list-covariant f g (cons x xs) = cong (\li -> cons (g (f x)) li) (list-covariant f g xs)
+
+
+list-is-functor : {l : Level} -> Functor List
+list-is-functor {l} = record { fmap        = list-fmap ;
+                               preserve-id = list-preserve-id ;
+                               covariant   = list-covariant {l}}
+
+--open module FunctorWithImplicits {l ll : Level} {F : Set l -> Set ll} {{functorT : Functor F}} = Functor functorT
+
+
+--hoge : ∀{F A} {{Functor F}} -> (Fa : F A) -> Functor.fmap id Fa ≡ id Fa
+--hoge = {!!}
+
+
+{-
+record NaturalTransformation {l ll : Level} (F G : Set l -> Set ll) : Set (suc (l ⊔ ll)) where
+  field
+    natural : {A : Set l}  -> F A -> G A
+  field
+    lemma : ∀{f } {x : Functor F} -> natural (fmap f x) ≡ f (natural x)
+-}```