[英]In a Haskell script, how does one programatically obtain the type signature of a function?
在Haskell(GHC)中,如何获得下面所示功能列表的类型签名?
[tail,init,reverse]
我尝试使用Data.Typeable
模块的typeOf
函数失败。 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会出现以下错误:
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测试用例不是我想要的,但我确实通过了(基于David Young的建议)。 该测试用例至少迫使编译器确认[tail,init,reverse]
的类型为[ [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
您不需要单元测试即可检查函数的类型。 单元测试是在代码编译后运行的,这是一个动态测试。 但是,类型检查是一种静态测试:所有类型都在程序编译期间进行测试。 因此,我们可以将GHC用作最小的静态类型检查器,并将您的程序简化为:
main :: IO ()
main = return ()
where
tailInitReverseAreListFunctions :: [[a] -> [a]]
tailInitReverseAreListFunctions = [tail, init, reverse]
在使用实际数据实际测试函数时,您甚至不需要测试,因为该应用程序也将(静态)测试函数的类型。
请记住,Haskell是一种静态类型的语言。 在运行代码之前 ,在编译过程中检查类型。 因此,任何类型检查单元测试或多或少都是代码气味,因为它只能通过。
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