在Haskell脚本中,如何以编程方式获取函数的类型签名？

Question

在Haskell(GHC)中,如何获得下面显示的函数列表的类型签名？

[tail,init,reverse]

我尝试使用模块的typeOf功能失败了Data.Typeable.具体来说,我尝试运行以下Haskell脚本:

import Data.Typeable
import Test.HUnit
myTest = TestCase
          ( assertEqual "\n\nShould have been \"[[a] -> [a]]\""
            "[[a] -> [a]]"
            (show ( typeOf [tail,init,reverse] )) )
tests = TestList [ (TestLabel "myTest" myTest) ]

但是,GHC响应以下错误:

C:\>ghci my_script.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( my_script.hs, interpreted )
my_script.hs:7:21: error:
    * No instance for (Typeable a0) arising from a use of `typeOf'
    * In the first argument of `show', namely
        `(typeOf [tail, init, reverse])'
      In the third argument of `assertEqual', namely
        `(show (typeOf [tail, init, reverse]))'
      In the first argument of `TestCase', namely
        `(assertEqual
            "\n\
            \\n\
            \Should have been \"[[a] -> [a]]\""
            "[[a] -> [a]]"
            (show (typeOf [tail, init, reverse])))'
Failed, modules loaded: none.
Prelude>

---

更新:以下HUnit测试用例并不是我想要的,但我确实得到了它(基于David Young的建议).此测试用例至少会强制编译器确认其[tail,init,reverse]类型[ [a] -> [a] ].

import Data.Typeable
import Test.HUnit
myTest = TestCase
          ( assertEqual "\n\nShould have been 3"
            3
            ( length ( [tail,init,reverse] :: [[a]->[a]] ) ) )
tests = TestList [ (TestLabel "myTest" myTest) ]

C:\>my_script.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( my_script.hs, interpreted )
Ok, modules loaded: Main.
*Main> runTestTT tests
Cases: 1  Tried: 1  Errors: 0  Failures: 0

Answer 1

您不需要单元测试来检查函数的类型.代码编译后运行单元测试,这是一个动态测试.但是,类型检查是一种静态测试:在编译程序期间测试所有类型.因此,我们可以使用GHC作为最小静态类型检查器,并将您的程序减少到:

main :: IO ()
main = return ()
  where
    tailInitReverseAreListFunctions :: [[a] -> [a]]
    tailInitReverseAreListFunctions = [tail, init, reverse]

当您使用真实数据实际测试函数时,您甚至不再需要该测试,因为该应用程序也将(静态地)测试函数的类型.

请记住,Haskell是一种静态类型语言.在编译期间,在运行代码之前检查类型.因此,任何类型检查单元测试或多或少都是代码味道,因为它只能通过.