例：

f :: Int -> String -> Bool
f x y = if (x > 10) then True else (g y)

g :: String -> Bool
g s = _

汇编：

Prelude> :l HoleEx.hs
[1 of 1] Compiling Main             ( HoleEx.hs,interpreted )

HoleEx.hs:6:7:
    Found hole `_' with type: Bool
    Relevant bindings include
      s :: String (bound at HoleEx.hs:6:3)
      g :: String -> Bool (bound at HoleEx.hs:6:1)
    In the expression: _
    In an equation for `g': g s = _
Failed,modules loaded: none.

在Scala中编程时,我通常使用???作为我的Scala代码中的占位符,我还没写过.

但是,使用打字孔对我来说似乎更强大.请注意,我可以用else替换上面的其他(g y)(g 55),但是我得到一个编译时错误：

Prelude> :l HoleEx.hs
[1 of 1] Compiling Main             ( HoleEx.hs,interpreted )

HoleEx.hs:3:39:
    Couldn't match type `Int' with `[Char]'
    Expected type: String
      Actual type: Int
    In the first argument of `g',namely `x'
    In the expression: (g x)
Failed,modules loaded: none.

虽然我可以使用???在Scala中获取占位符实现进行编译,与孔不同,我将使用???获取运行时错误.

scala> def g(x: Int): Int = ???
g: (x: Int)Int

scala> g(5)
scala.NotImplementedError: an implementation is missing

Scala是否有打孔？