Please see this newly written code now, there is no "read" used, and still I get error on ambiguous "show":
data MyType0 a = Tong1 a | Tong2 a deriving Show
data MyType1 a = Cons1 a | Cons2 a | Cons3 | Cons4 deriving Show
data MyType2 a = MyType2 a deriving Show
fun ((Cons2 s):t:ts) (symseq, MyType2 msg) = (symseq, MyType2 (msg ++ ["You provided wrong symbol: " ++ (show t) ++ " Please provide symbol: " ++ (show Cons3) ]))
--fun ((Cons2 s):t:ts) (symseq, MyType2 msg) = (symseq, MyType2 (msg ++ ["You provided wrong symbol: " ++ (show t) ++ " Please provide symbol: " ]))
fun _ syms = syms
The ghci error msg:
showerr.hs:6:148:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `show'
Probable fix: add a type signature that fixes these type variable(s)
In the second argument of `(++)', namely `(show Cons3)'
In the second argument of `(++)', namely
`" Please provide symbol: " ++ (show Cons3)'
In the second argument of `(++)', namely
`(show t) ++ " Please provide symbol: " ++ (show Cons3)'
Failed, modules loaded: none.
note that the commented part doesn't give this error. please explain why this is so error.
original message is kept below.
I got the following code on a site. I know ambiguous type error can come for the "read" function but here i get it for the "show" function too. i find it strange and not understand.
code:
main = do run <- getLine
val <- getLine
case run of
"len" -> print . show . len $ (read val)
"rev" -> print . show . rev $ (read val)
_ -> putStr "wrong option"
rev :: [a] -> [a]
rev = foldl (flip (:)) []
len :: [a] -> Int
len = foldl (\ac _ -> ac + 1) 0
When i load it in ghci i get error.
ideone_x0cMx.hs:4:46:
Ambiguous type variable `a0' in the constraint:
(Read a0) arising from a use of `read'
Probable fix: add a type signature that fixes these type variable(s)
In the second argument of `($)', namely `(read val)'
In the expression: print . show . len $ (read val)
In a case alternative: "len" -> print . show . len $ (read val)
ideone_x0cMx.hs:5:46:
Ambiguous type variable `a1' in the constraints:
(Read a1) arising from a use of `read' at ideone_x0cMx.hs:5:46-49
(Show a1) arising from a use of `show' at ideone_x0cMx.hs:5:32-35
Probable fix: add a type signature that fixes these type variable(s)
In the second argument of `($)', namely `(read val)'
In the expression: print . show . rev $ (read val)
In a case alternative: "rev" -> print . show . rev $ (read val)
Failed, modules loaded: none.