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Haskell

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group by key in haskell

Haskell
Scala
Programming

什麼是 Group By Key

Group By Key 就是把 Element 計算一個 key，相同 key 的 element 就放左一組內 例如 group by key by the reminder equal to 3 in Scala

val arr = Array(2, 4, 5, 6, 9, 23, 24, 25, 31, 37)
scala> val ans = arr.groupBy { n => n % 3 }
ans: scala.collection.immutable.Map[Int,Array[Int]] = Map(2 -> Array(2, 5, 23), 1 -> Array(4, 25, 31, 37), 0 -> Array(6, 9, 24))

問題

Haskell 中把 List 中的 element Group By Key 是麻煩的一件事 因為 Data.List.groupBy…

Exception: Prelude.read: no parse

Haskell

    solveRPN' :: (Num a, Read a) => String -> a  
    solveRPN' = head . foldl foldingFunction [] . words  
        where   foldingFunction (x:y:ys) "*" = (x * y):ys  
                foldingFunction (x:y:ys) "+" = (x + y):ys  
                foldingFunction (x:y:ys) "-" = (y - x):ys  
                foldingFunction xs numberString = read numberString:xs

    solveRPN' "10 4.0 3 + 2 * -"
    *** Exception: Prelude.read: no parse

By default, the ghc will deduce this function as (Num Int, Read Int) . The error is casued by the Prelude.read function which cannot parse a "Floating number string…