hayaのHaskell日記

2006-08-28

[] 11.2節 Maybeモナド 00:35

Maybeモナドの定義

data Maybe a = Nothing | Just a deriving (Eq, Ord)

instance Monad Maybe where
  (Just x) >>= f  = f x
  Nothing  >>= f  = Nothing
  
  return x        = Just x

タプルのリストをlookup

lookup :: a -> [(a, b)] -> Just b

Maybeモナドとlookupを組み合わせると、RubyのHashっぽいことができる。

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