在Haskell中是否存在非递归绑定的语法，就像`let`和`let rec`在类似语言中的区别？ [英] is there any syntax for non-recursive binding in Haskell, just like the difference between `let` and `let rec` in similar languages?

问题描述

非递归绑定允许我限制绑定值，例如：

Non-recursive binding allows me to shadow bound value, for instance:

b a = let norec a = a + 10 in a

这里让norec 意味着 let 绑定但不能递归。

here let norec created by myself means a let binding but not recursive.

使用记录通配符时非常有用：

This is extremely helpful when using record wildcards:

data MyRecord = MyRecord{ {- vary huuuuuge set of definitions -} }

foo MyRecord{..} = let norec field1 = field1 + 1
                             field2 = modify field2
                             {- some other modifications to the fields -}
                    in MyRecord{..}

可以实现吗？或者你如何处理你的情况？

Is that achievable? Or how do you deal with it in your cases?

推荐答案

实际上记录通配符在这里有用吗？通常的做事方式对我来说看起来相当简洁：

Are record wildcards actually that useful here? The usual old way of doing things looks quite concise to me:

foo r = r { field1 = field1 r + 1, field2 = modify (field2 r) }

直接回答你的问题是没有非递归Haskell中的 let 的模拟;尽管你可以使用 Identity monad来排序：

The direct answer to your question is that there is no non-recursive analog of let in Haskell; though you can use the Identity monad to sort of hack something into place:

foo MyRecord{..} = runIdentity $ do
    field1 <- return (field1 + 1)
    field2 <- return (modify field2)
    return MyRecord{..}

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