haskell - 我怎么知道我没有完全应用这个 ghc 错误消息中的函数？

Question

我从 ghc 收到一条我没有理解的错误消息，并将我的代码简化为:

import System.Process

main = do
  (_, out, _) <- readProcessWithExitCode "echo" ["foo"]
  putStr out

(我应该给 readProcessWithExitCode 一个额外的参数)。使用 runghc 编译该损坏的程序给出:

Test.hs:4:2:
    Couldn't match expected type `IO
                                    (GHC.IO.Exception.ExitCode, String, String)'
           against inferred type `(a, b, c)'
    In the pattern: (_, out, _)
    In a stmt of a 'do' expression:
        (_, out, _) <- readProcessWithExitCode "echo" ["foo"]
    In the expression:
        do { (_, out, _) <- readProcessWithExitCode "echo" ["foo"];
             putStr out }

我应该如何从这个 ghc 错误消息中发现我未能完全应用函数？

Answer 1

如果你指定了类型签名main::IO()，那么错误就会变成:

test.hs:5:18:
    Couldn't match expected type `IO t0'
                with actual type `String
                                  -> IO (GHC.IO.Exception.ExitCode, String, String)'
    In the return type of a call of `readProcessWithExitCode'
    In a stmt of a 'do' expression:
        (_, out, _) <- readProcessWithExitCode "echo" ["foo"]
    In the expression:
      do { (_, out, _) <- readProcessWithExitCode "echo" ["foo"];
           putStr out }

在这种情况下，错误会更加明显。

我的猜测是错误不是很清楚，因为在推断类型时，GHC 认为您在函数中(即 (a->)) monad 而不是 IO，其中万一你之后会应用这个论点。

所以不要偷懒，包括那些类型签名! ;-)