RankNpolymorphism和kleisli箭头的离谱财富 [英] RankNpolymorphism and kleisli arrows of outrageous fortune
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问题描述
我不明白为什么demonbind1的定义会产生一些编译错误。它看起来像一个愚蠢的翻转,但不知何故......
{ - #LANGUAGE GADTs# - }
pre>
{ - #LANGUAGE RankNTypes,ScopedTypeVariables,TypeOperators,TypeFamilies,ImpredicativeTypes# - }
类型a: - > b =全部我。一个i - > b i
class IFunctor f where imap ::(a: - > b) - > (f a: - > f b)
class(IFunctor m)=> IMonad m其中
skip :: a: - > m a
bind ::(a: - > m b) - > (m a: - > m b)
- Conor McBride的恶魔绑定
(?> =):: forall m a b i。 (IFunctor m,IMonad m)=> m a i - > (a: - > m b) - > mbi
(?> =)=
let
- OK
demonbind0 = flip(bind :: forall i。(forall j。aj - > mbj) - > ; mai - > mbi)
- KO - 在
下面看到错误demonbind1 = flip bind :: forall i。 m a i - > (所有j,a j - > m b j) - > m b我
- 所以我必须写这个
demonbind2 :: forall i。 (mai - >(a: - > mb) - > mbi)
demonbind2 mai ti =(bind ti)mai
in demonbind2
错误是
无法匹配类型'a j0 - > m b j0'...
与'forall i2。一个i2 - > (a j0→m b j0)→ m a i1 - > m b i1
实际类型:a: - > m b - > m a i1 - > m b i1
在'flip'的第一个参数中,即'bind'
在表达式中:
flip bind :: forall i。 m a i - > (所有j,a j - > m b j) - > mbi
解决方案有点令人惊讶的是,
ImpredicativeTypes
在GHC 8.0的开发快照中似乎比平时更少破碎!这个编译没有错误:
(?> =)::(IFunctor m,IMonad m)=> m a i - > (a: - > m b) - > mbi
(?> =)=翻转绑定
。
I dont understand why the definition of demonbind1 yields some compiler errors. it looks like a stupid flip but somehow..
{-# LANGUAGE GADTs #-} {-# LANGUAGE RankNTypes, ScopedTypeVariables, TypeOperators, TypeFamilies,ImpredicativeTypes #-} type a :-> b = forall i . a i -> b i class IFunctor f where imap :: (a :-> b) -> (f a :-> f b) class (IFunctor m) => IMonad m where skip :: a :-> m a bind :: (a :-> m b) -> (m a :-> m b) -- Conor McBride's "demonic bind" (?>=) :: forall m a b i. (IFunctor m, IMonad m) => m a i -> (a :-> m b) -> m b i (?>=) = let -- OK demonbind0 = flip (bind :: forall i. (forall j. a j -> m b j) -> m a i -> m b i ) -- KO - see error below demonbind1 = flip bind :: forall i. m a i -> (forall j. a j -> m b j) -> m b i -- So i have to write this demonbind2 :: forall i. (m a i -> (a :-> m b) -> m b i ) demonbind2 mai ti = (bind ti) mai in demonbind2
The error is
Couldn't match type ‘a j0 -> m b j0’ … with ‘forall i2. a i2 -> m b i2’ Expected type: (a j0 -> m b j0) -> m a i1 -> m b i1 Actual type: a :-> m b -> m a i1 -> m b i1 In the first argument of ‘flip’, namely ‘bind’ In the expression: flip bind :: forall i. m a i -> (forall j. a j -> m b j) -> m b i
解决方案Somewhat surprisingly,
ImpredicativeTypes
seem less broken than usual on the development snapshot of GHC 8.0! This compiles without errors:(?>=) :: (IFunctor m, IMonad m) => m a i -> (a :-> m b) -> m b i (?>=) = flip bind
I wonder what change fixed the problem.
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