英文:
calculate function return type for a given input type
问题
在Haskell中，我想计算给定函数的返回类型，假设已知参数类型。例如，对于具有签名a -> a的函数和参数类型Bool，我想获得返回类型Bool。现在，这个方法可以工作：
-- $ ghci -XScopedTypeVariables
> import Test.QuickCheck
> import Data.Typeable
> fn = id
> param :: Bool <- generate arbitrary
> typeOf $ fn param
Bool
但是，必须实例化参数类型然后执行函数感觉有点繁琐，如果目标只是获取其返回类型的话。
我想象中，如果知道参数类型Bool，我们将知道类型变量a也将变成Bool，因此我们可以将其替换到返回类型a中，以获得返回类型Bool。是否有一种方法可以动态获取这样的函数返回类型，而不必执行函数？
英文:
In Haskell, I would like to calculate the return type for a given function given a parameter type.
So for e.g. a function with signature a -> a and a parameter type Bool, I would like to get the return type Bool.
Now, this works:
-- $ ghci -XScopedTypeVariables
> import Test.QuickCheck
> import Data.Typeable
> fn = id
> param :: Bool <- generate arbitrary
> typeOf $ fn param
Bool
However, having to instantiate the parameter type then execute the function feels a bit overkill, if the goal is just to obtain its return type.
I imagine that, knowing the param type Bool, we would know the type variable a would also become Bool, and that therefore we can substitute this in the return type a as well, to obtain return type Bool.
Is there a way to dynamically obtain such a function return type without having to resort to executing the function?
答案1
得分: 5
请注意，typeOf 不会评估其参数，因此您发布的代码实际上从未调用过 fn。
更简洁地说，您甚至可以使用 typeOf $ fn (undefined :: Bool) 将底部参数传递给 fn，而不触发异常。
话虽如此，我无法理解您为什么认为需要在运行时计算类型。在 Haskell 中很少需要这样做。您的实际目标是什么？
英文:
Note that typeOf does not evaluate its argument, so your posted code never actually calls fn.
More succinctly, one can even use typeOf $ fn (undefined :: Bool) to pass a bottom argument to fn without triggering an exception.
That being said, I can not understand why you think you need to compute types a runtime. This is very rarely needed in Haskell. What is your actual goal?
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