haskell - 如何在 GHCi 中使用多个 where 子句？

Question

我第一次玩 GHCi，我在编写多行函数时遇到了一些麻烦。
我的代码如下:

Prelude> :{
Prelude| let diffSquares lst = abs $ squareOfSums lst - sumOfSquares lst
Prelude|   where
Prelude|     squareOfSums lst = (fst (sumsAndSquares lst))^2
Prelude|     sumOfSquares lst = snd (sumsAndSquares lst)
Prelude|     sumsAndSquares = foldl (\(sms,sqrs) x -> (sms+x,sqrs+x^2)) (0,0)
Prelude| :}

它给出了以下错误:

<interactive>:1:142: parse error on input `='

有人可以指出我所缺少的方向吗？

Answer 1

来自 ghci 的帮助手册( http://www.haskell.org/ghc/docs/6.10.4/html/users_guide/interactive-evaluation.html ):

Such multiline commands can be used with any GHCi command, and the lines between :{ and :} are simply merged into a single line for interpretation. That implies that each such group must form a single valid command when merged, and that no layout rule is used.

因此，您必须在每个定义之间插入分号，例如

Prelude> :{
Prelude| let a x = g
Prelude|   where
Prelude|     g = p x x;      {- # <----- # -}
Prelude|     p a b = a + b
Prelude| :}

编辑:在最新版本的 GHCi 中，您似乎需要一对大括号。

Prelude> :{
Prelude| let { a x = g
Prelude|   where
Prelude|     g = p x x
Prelude|     p a b = a + b
Prelude| }
Prelude| :}
Prelude> a 5
10