在表达中绑定

Question

我正在尝试将带有文件名的wheatear参数传递给解析文件或从标准输入读取：

let formulae = fmap parseInput $ if isInputFile args then (hGetContents $ openFile (last args) ReadMode) else hGetContents stdin

但当然会发生以下错误：

formula-2-bdd.hs:89:79:
    Couldn't match expected type `Handle' with actual type `IO Handle'
    In the second argument of `($)', namely
      `openFile (last args) ReadMode'
    In the expression: (hGetContents $ openFile (last args) ReadMode)

如果我先绑定IO Handle，我就不能根据`isInputFile args：

来决定

handle <- openFile (last args) ReadMode
formulae = fmap parseInput $ if isInputFile args then hGetContents handle else hGetContents stdin