Question

我有两个模块定义如下：

Foo.hs

module Foo
where

class F t where
  c :: t -> Char

instance F Double where
  c s = 'x'

Main.hs

import Foo

main :: IO ()
main =  do
  print $ (c 2.0)

当我编译这两个模块时，我得到了这个错误：

ghc Foo.hs Main.hs 
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )

Main.hs:6:12: error:
    • Ambiguous type variable ‘t0’ arising from a use of ‘c’
      prevents the constraint ‘(F t0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘t0’ should be.
      These potential instance exist:
        instance [safe] F Double -- Defined in ‘Foo’
    • In the second argument of ‘($)’, namely ‘(c 2.0)’
      In a stmt of a 'do' block: print $ (c 2.0)
      In the expression: do { print $ (c 2.0) }

Main.hs:6:14: error:
    • Ambiguous type variable ‘t0’ arising from the literal ‘2.0’
      prevents the constraint ‘(Fractional t0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘t0’ should be.
      These potential instances exist:
        instance Fractional Double -- Defined in ‘GHC.Float’
        instance Fractional Float -- Defined in ‘GHC.Float’
        ...plus one instance involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the first argument of ‘c’, namely ‘2.0’
      In the second argument of ‘($)’, namely ‘(c 2.0)’
      In a stmt of a 'do' block: print $ (c 2.0)

如何解决此问题，以便打印出'x'？

Answer 1

您应该将evaluate重命名为call。

编辑：尝试将c注释为2.0：

Double

GHC不知道print $ c (2.0 :: Double)使用哪种类型，它可以是任何2.0类型。要解决此问题，我们会通过明确标记其类型来强制Fractional为2.0类型。

在main方法中打印出一个char

1 个答案: