s.Count（Char.IsLetter）有什么问题 [英] Whats wrong with s.Count(Char.IsLetter)

问题描述

F＃

let s = "bugs 42 bunny"
s.Count(fun c -> Char.IsLetter(c))
s.Where(fun c -> Char.IsLetter(c)).ToArray()
s.Where(Char.IsLetter).ToArray()
s.Count(Char.IsLetter) // error

为什么仅最后一行无法编译：

错误FS0002：该函数接受了太多参数，或者在没有该函数的情况下使用预期的

Error FS0002: This function takes too many arguments, or is used in a context where a function is not expected

推荐答案

我认为这是类型推断的一个极端案例，成员重载。 Count 和 Where 之间的区别在于，前者有两个带有不同数量参数的重载。

I think it's an edge case of type inference wrt member overloading. The difference between Count and Where is that the former has two overloads with different number of arguments.

您可以通过指定从F＃函数到 System.Func< _，_> ：

You can work around by specifying conversion from F# function to System.Func<_, _>:

s.Count(Func<_, _>(Char.IsLetter))

当然，它比相应的版本还要丑：

Of course, it's even uglier than the corresponding version:

s.Count(fun c -> Char.IsLetter(c))

您可以在 https://visualfsharp.codeplex.com/workitem/list/basic ，以便对其进行修复

You could file a bug at https://visualfsharp.codeplex.com/workitem/list/basic so that it may be fixed in the F# vNext.

请注意，在F＃中，您并不经常使用Linq函数。您可以执行以下操作：

Note that in F#, you don't often use Linq functions. You can either do:

s |> Seq.sumBy (fun c -> if Char.IsLetter c then 1 else 0)

或

s |> Seq.filter Char.IsLetter |> Seq.length

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