haskell - HSpec Nothing expectation failing to compile -

i'm learning haskell , i've written function:

safehead :: [a] -> maybe safehead [] = nothing safehead (x:xs) = x 

i'm trying test hspec:

import test.hspec  main :: io () main = hspec spec  spec :: spec spec =    describe "safehead" $     "should return nothing empty list" $       safehead [] `shouldbe` nothing 

but fails compile:

error:(14, 19) ghc: no instance (eq a0) arising use of ‘shouldbe’     type variable ‘a0’ ambiguous     note: there several potential instances:       instance eq => eq (maybe a) -- defined in ‘ghc.base’       instance eq => eq (ghc.real.ratio a) -- defined in ‘ghc.real’       instance eq ordering -- defined in ‘ghc-prim-0.4.0.0:ghc.classes’       ...plus 31 others     in second argument of ‘($)’, namely       ‘safehead [] `shouldbe` nothing’     in second argument of ‘($)’, namely       ‘it "should return nothing empty list"        $ safehead [] `shouldbe` nothing’     in expression:       describe "safehead"       $ "should return nothing empty list"         $ safehead [] `shouldbe` nothing 

i have tried this:

safehead :: (eq a) => [a] -> maybe safehead [] = nothing safehead (x:xs) = x 

but still fails with:

error:(14, 19) ghc: no instance (eq a0) arising use of ‘shouldbe’     type variable ‘a0’ ambiguous     note: there several potential instances:       instance (eq a, eq b) => eq (either b)         -- defined in ‘data.either’       instance eq data.monoid.all -- defined in ‘data.monoid’       instance forall (k :: box) (f :: k -> *) (a :: k).                eq (f a) =>                eq (data.monoid.alt f a)         -- defined in ‘data.monoid’       ...plus 43 others     in second argument of ‘($)’, namely       ‘safehead [] `shouldbe` nothing’     in second argument of ‘($)’, namely       ‘it "should return nothing empty list"        $ safehead [] `shouldbe` nothing’     in expression:       describe "safehead"       $ "should return nothing empty list"         $ safehead [] `shouldbe` nothing 

i don't know what's problem here. if try other tests these, compiles fine:

    "should return head" $       safehead [1] `shouldbe` 1       safehead [2,3,4,5,6,1] `shouldbe` 2 

so, it's nothing itself, cannot compared equals? how assert returns nothing then? or function generic?

side note: i've seen similar error function:

palindrome :: (eq a) => [a] -> [a] palindrome xs = xs ++ reverse xs 

when trying test empty lists:

palindrome [] `shouldbe` [] 

which fails with:

error:(26, 21) ghc: no instance (eq a0) arising use of ‘shouldbe’     type variable ‘a0’ ambiguous     note: there several potential instances:       instance eq => eq (maybe a) -- defined in ‘ghc.base’       instance eq => eq (ghc.real.ratio a) -- defined in ‘ghc.real’       instance eq ordering -- defined in ‘ghc-prim-0.4.0.0:ghc.classes’       ...plus 32 others     in stmt of 'do' block: palindrome [] `shouldbe` []     in second argument of ‘($)’, namely       ‘do { palindrome [] `shouldbe` [] }’     in second argument of ‘($)’, namely       ‘it          "should turn list palindrome, reads same both forwards , backwards"        $ { palindrome [] `shouldbe` [] }’ 

so, it's nothing itself, cannot compared equals?

what's nothing's type? it's nothing :: maybe a. , ghc doesn't a in context: "the type variable ‘a0’ ambiguous". after all, shouldbe takes can compared (==) , shown. , maybe a instance of eq if a instance of eq. ghc can't possibly know which a want use, need specify hand:

  describe "safehead" $     "should return nothing empty list" $       safehead [] `shouldbe` (nothing :: maybe int) 

that's not coercion, you're making clear which of possible types want use. other examples:

  describe "safehead" $     "should return nothing empty list" $       safehead [] `shouldbe` (nothing :: maybe int)       safehead [] `shouldbe` (nothing :: maybe ())       safehead [] `shouldbe` (nothing :: maybe integer)       safehead [] `shouldbe` (nothing :: maybe char)