autotaker
Haskellで型クラス実装されてなくても動くような、いわゆるSoft Constraintsの言語拡張って難しいんでしょうか？
(!) :: (Show? key, Ord key) => Map key value -> key -> value
(!) m key = case lookup key m of
Just value -> value
Nothing -> case show? of
Nothing -> error $ "no value for key"
Just fshow -> error $ "no value for key: "++ fshow key
>>> empty ! "hoge"
Error: no value for key: "hoge"
>>> newtype T = T Int deriving(Eq,Ord)
>>> empty ! (T 10)
Error: no value for key