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do記法でパターンマッチにしくじるとfailが使われる

Haskell

Last updated at 2018-07-31Posted at 2012-12-26

以下のhogeとhoge'は同じかと思ってたけど、別物。

hoge :: [Int]
hoge = do
  Just x <- [Just 3, Just 2, Nothing, Just 4]
  return x

hoge' :: [Int]
hoge' = [Just 3, Just 2, Nothing, Just 4] >>= (\ (Just x) -> return x)

http://www.haskell.org/onlinereport/exps.html によると、do記法の書き換えは以下なので。

do {p <- e; stmts}  =   let ok p = do {stmts}
                            ok _ = fail "..."
                        in e >>= ok

動きから予想はしてたけど、やっぱ fail かあ。。。

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