Question

我开始做99个haskell问题而且我在problem 7并且我的单元测试正在爆炸。

显然，这是由于：http://www.haskell.org/haskellwiki/Monomorphism_restriction

我只是想确保我理解正确，因为我有点困惑。

情况1：func a定义为没有类型def或非严格类型def然后使用一次，编译器在编译时没有问题推断类型。

情况2：在程序中多次使用相同的func a，编译器不能100％确定类型是什么，除非它重新计算给定参数的函数。

为了避免计算损失，ghc向程序员抱怨它需要a上的严格类型def 工作正常。

我认为在我的情况下，assertEqual的类型为

 assertEqual :: (Eq a, Show a) => String -> a -> a -> Assertion

我在定义test3时遇到错误，我解释为它有两种可能的类型，用于返回testcase3（显示和均衡）并且不知道如何继续。

听起来是否正确还是完全关闭？

problem7.hs：

-- # Problem 7
-- Flatten a nested list structure.

import Test.HUnit

-- Solution

data NestedList a = Elem a | List [NestedList a]

flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List x) = concatMap flatten x

-- Tests

testcase1 = flatten (Elem 5)
assertion1 = [5]

testcase2 = flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])
assertion2 = [1,2,3,4,5]

-- This explodes
-- testcase3 = flatten (List [])

-- so does this:
-- testcase3' = flatten (List []) :: Eq a => [a]

-- this does not
testcase3'' = flatten (List []) :: Num a => [a]

-- type def based off `:t assertEqual`
assertEmptyList :: (Eq a, Show a) => String -> [a] -> Assertion
assertEmptyList str xs = assertEqual str xs []

test1 = TestCase $ assertEqual "" testcase1 assertion1
test2 = TestCase $ assertEqual "" testcase2 assertion2
test3 = TestCase $ assertEmptyList "" testcase3''

tests = TestList [test1, test2, test3]

-- Main
main = runTestTT tests

第一种情况：testcase3 = flatten (List [])

GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( problem7.hs, interpreted )

problem7.hs:29:20:
    Ambiguous type variable `a0' in the constraints:
      (Eq a0)
        arising from a use of `assertEmptyList' at problem7.hs:29:20-34
      (Show a0)
        arising from a use of `assertEmptyList' at problem7.hs:29:20-34
    Probable fix: add a type signature that fixes these type variable(s)
    In the second argument of `($)', namely
      `assertEmptyList "" testcase3'
    In the expression: TestCase $ assertEmptyList "" testcase3
    In an equation for `test3':
        test3 = TestCase $ assertEmptyList "" testcase3
Failed, modules loaded: none.
Prelude>

第二种情况：testcase3 = flatten (List []) :: Eq a => [a]

GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( problem7.hs, interpreted )

problem7.hs:22:13:
    Ambiguous type variable `a0' in the constraints:
      (Eq a0)
        arising from an expression type signature at problem7.hs:22:13-44
      (Show a0)
        arising from a use of `assertEmptyList' at problem7.hs:29:20-34
    Possible cause: the monomorphism restriction applied to the following:
      testcase3 :: [a0] (bound at problem7.hs:22:1)
    Probable fix: give these definition(s) an explicit type signature
                  or use -XNoMonomorphismRestriction
    In the expression: flatten (List []) :: Eq a => [a]
    In an equation for `testcase3':
        testcase3 = flatten (List []) :: Eq a => [a]
Failed, modules loaded: none.

Answer 1

并不是单形态限制，而是由defaulting解决的模糊类型变量会导致编译失败。

-- This explodes
-- testcase3 = flatten (List [])

-- so does this:
-- testcase3' = flatten (List []) :: Eq a => [a]

-- this does not
testcase3'' = flatten (List []) :: Num a => [a]

flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List x) = concatMap flatten x

flatten对类型变量a没有约束，因此testcase3的定义没有问题，因为它会是多态的。

但是当你在test3中使用它时，

test3 = TestCase $ assertEmptyList "" testcase3 -- ''

你继承了

的约束

assertEmptyList :: (Eq a, Show a) => String -> [a] -> Assertion

现在编译器必须找出应该在那里使用哪种类型testcase3。没有足够的上下文来确定类型，因此编译器尝试通过默认来解析类型变量。根据{{3}}，无法通过默认来解决上下文(Eq a, Show a)，因为只有涉及至少一个数字类的上下文才有资格进行默认。因此，由于模糊的类型变量，编译失败了。

然而，

testcase3'和testcase3''属于单态限制，因为表达式类型签名会对左侧继承的定义的右侧施加约束。

testcase3'无法编译，因为无论是否在断言中使用它。

由于表达式类型签名强加了数字约束，因此

testcase3''默认为[Integer]。因此，当testcase''的类型为单态时，约束类型变量默认为Integer。然后，test3中没有问题的类型。

如果您已经为绑定而不是右侧提供了类型签名，

testcase3' :: Eq a => [a]
testcase3' = flatten (List [])

testcase3'' :: Num a => [a]
testcase3'' = flatten (List [])

这两个值都会自行编译为多态值，但testcase3''只能使用test3，因为只有这样才会引入所需的数字约束以允许默认值。

请向我解释单态限制吗？

1 个答案: