在GHCi中定义功能签名 [英] Defining function signature in GHCi

问题描述

在Haskell的解释器GHCi中定义功能签名不起作用.复制此页面中的示例:

Defining a function signature in Haskell's interpreter GHCi doesn't work. Copying an example from this page:

Prelude> square :: Int -> Int

<interactive>:60:1: error:
    • No instance for (Show (Int -> Int)) arising from a use of ‘print’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of an interactive GHCi command: print it
Prelude> square x = x * x

我如何声明一个函数签名，然后在Haskell中交互地给出函数定义?还:为什么我不能简单地对函数进行求值并在定义后查看其类型(例如Prelude> square)?

How can I declare a function signature and then give function definition in Haskell interactively? also: why can't I simply evaluate the function and see its type (e.g. Prelude> square) once it has been defined?

推荐答案

您可以在ghc交互式外壳中定义函数签名.但是，问题在于您需要在单个命令中定义功能.

You can define a function signature in the ghc interactive shell. The problem however is that you need to define functions in a single command.

您可以使用分号(;)在两个部分之间进行分割:

You can use a semicolon (;) to split between two parts:

Prelude> square :: Int -> Int; square x = x * x

请注意，具有多个子句的功能也是如此.如果您写:

Note that the same holds for a function with multiple clauses. If you write:

Prelude> is_empty [] = True
Prelude> is_empty (_:_) = False

您实际上已经用第二条语句覆盖了先前的is_empty函数.如果然后使用空列表进行查询，则会得到:

You have actually overwritten the previous is_empty function with the second statement. If we then query with an empty list, we get:

Prelude> is_empty []
*** Exception: <interactive>:4:1-22: Non-exhaustive patterns in function is_empty

所以ghci将最后一个定义作为单子句函数定义.

So ghci took the last definition as a single clause function definition.

同样，您必须这样写:

Prelude> is_empty[] = True; is_empty (_:_) = False

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