Question

我写了一个非常简单的Haskell程序：

main = print $ sum $ map read ["55", "99", "101"]

根据我过去的经验，我预计会出现“模糊类型”错误，因为sum $ map read [...]的签名是(Read a, Num a) => a; Num是一个类，因此本身不能实现Show类。但是，程序正确输出“255”。 print如何确定产生输出的方法？（show也能够生成正确的结果且没有错误。）

Answer 1

如果你使用-fwarn-type-defaults选项，你会得到这个：

$ ghc -O2 -fwarn-type-defaults ddd.hs
[1 of 1] Compiling Main             ( ddd.hs, ddd.o )

ddd.hs:2:8: Warning:
    Defaulting the following constraint(s) to type ‘Integer’
      (Show s0) arising from a use of ‘print’ at ddd.hs:2:8-12
      (Read s0) arising from a use of ‘read’ at ddd.hs:2:26-29
      (Num s0) arising from a use of ‘sum’ at ddd.hs:2:16-18
    In the expression: print
    In the expression: print $ sum $ map read ["55", "99", "101"]
    In an equation for ‘main’:
        main = print $ sum $ map read ["55", "99", "101"]

解释了发生了什么。

（缺少）在Haskell中使用Read和Show时的模糊类型

1 个答案: