[英]Haskell partial application doesn't seem to work with on. Why?
局部使用單個其他函數似乎可以更改所涉及的類型。
從比較長度“ aaa”,“ bb”開始,然后從右側剝離項目,事情開始時就可以預測:
Prelude Data.Function> :t on compare length "aaa" "bb"
on compare length "aaa" "bb" :: Ordering
Prelude Data.Function> on compare length "aaa" "bb"
GT
Prelude Data.Function> :t on compare length "aaa"
on compare length "aaa" :: [Char] -> Ordering
Prelude Data.Function> let ocla = on compare length "aaa"
Prelude Data.Function> :t ocla
ocla :: [Char] -> Ordering
Prelude Data.Function> ocla "aa"
GT
Prelude Data.Function> :t on compare length
on compare length :: [a] -> [a] -> Ordering
Prelude Data.Function> let ocl = on compare length
Prelude Data.Function> :t ocl
ocl :: [a] -> [a] -> Ordering
Prelude Data.Function> ocl "aaa" "aa"
GT
但是相比之下,我感到驚訝:
Prelude Data.Function> :t on compare
on compare :: Ord b => (a -> b) -> a -> a -> Ordering
Prelude Data.Function> let oc = on compare
Prelude Data.Function> :t oc
oc :: (a -> ()) -> a -> a -> Ordering
Prelude Data.Function> oc length "aaa" "aa"
<interactive>:27:4:
Couldn't match type `Int' with `()'
Expected type: [Char] -> ()
Actual type: [Char] -> Int
In the first argument of `oc', namely `length'
In the expression: oc length "aaa" "aa"
In an equation for `it': it = oc length "aaa" "aa"
為什么oc的類型與on compare的類型不同?
這是由於可怕的單態性限制。 GHCi使用Monomorphism限制來猜測在交互模式下定義的函數或值的最簡單類型。 這通常是非常有用的,但是在您確實需要類型類約束類型的地方(例如Ord a => a
()
,它常常是笨拙的()
。 只要為您的oc
函數提供類型簽名,就可以了:
let oc :: Ord b => (a -> b) -> a -> a -> Ordering; oc = on compare
如果想使用:set -XNoMonomorphismRestriction
(感謝@Xeo),可以關閉Monomorphism限制,但我建議您反對。 您可以將函數放入文件中並將其加載到GHCi中,或者以交互方式添加類型簽名。
維基文章( http://www.haskell.org/haskellwiki/Monomorphism_restriction)更詳細地說明了此問題。
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