是否必須手動打開證據，prj'或者是否有更直接的方法來指導實體求解器？
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE UndecidableInstances #-}
module CatLib.SO.SO_TCMatchingDict where
class OneSide a where oneSide :: a
class (OneSide a {- , ManyOtherTC a -}) => TC a
instance (OneSide a {- , ManyOtherTC a -}) => TC a
class Other f where other :: forall a. f a
instance Other f => OneSide (f x) where oneSide = other
prj :: forall f a. (forall x. TC (f x)) => f a
prj = oneSide @(f a) -- Could not deduce (Other f) arising from a use of ‘oneSide’
prj' :: forall f g tag a b. (forall x. TC (f x)) => f a
prj' = case Dict :: Dict (TC (f a)) of
Dict -> oneSide @(f a) -- that's ok
data Dict c = c => Dict
uj5u.com熱心網友回復：
怎么樣
prj :: forall f a. (forall x. TC (f x)) => f a
prj = go
where
go :: TC (f a) => f a
go = oneSide @(f a)
或者，在一行中：
prj :: forall f a. (forall x. TC (f x)) => f a
prj = oneSide :: TC (f a) => f a
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