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我開始做99個哈斯克爾問題，我在problem 7，我的單位測試結果爆炸了。請向我解釋單態限制嗎？
顯然，這是由於這樣的：http://www.haskell.org/haskellwiki/Monomorphism_restriction
我只是想確保我正確地理解這一點，因爲我有點困惑。
情況1：func a被定義爲沒有類型def或非嚴格類型def，然後被使用一次，編譯器在編譯時沒有問題推斷類型。
情況2：相同FUNC a多次使用的程序，編譯器不能100％確定是什麼類型，除非它重新計算對於給定參數的函數。
爲了避免計算損失，ghc向程序員抱怨說，它需要一個嚴格的類型def在a 才能正常工作。
我想在我的情況，assertEqual有
assertEqual :: (Eq a, Show a) => String -> a -> a -> Assertion
我得到了一個錯誤的類型定義時test3定義，我解釋說，它有2種可能的類型爲testcase3回報（展會和公式），並不知道如何繼續。
這聽起來是對的還是我完全沒有？
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
月1日的情況：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>
第二情況：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.