[Haskell-beginners] Ambiguous type variable prevents the constraint `(Ord t0)' from being solved.

[Sylvain Henry]

Try removing the lambda:

applyd f d x =
     let apply_listd l d x = [..]
     in let k =  5 -- hash x - todo
     [...]

Regards,
Sylvain

On 28/01/2017 21:09, Ivan Kush wrote:
> I get this error (full message at the end of the mail). How could I correct my code?
>
>
> ===================
> Code:
> ===================
>
> module Intro where
>
> import Data.Bits --  for xor, .&.
>
> data Func a b
>      = Empty
>      | Leaf Int [(a, b)]
>      | Branch Int Int (Func a b) (Func a b)
>                  
> applyd =
>      let apply_listd l d x =
>                      case l of
>                          [] -> d x
>                          (a, b) : t ->
>                              let c = compare x a
>                              in if c == EQ then b
>                                  else if c == GT then apply_listd t d x
>                                  else d x
>                                      
>       in  \f d x ->
>          let k =  5 -- hash x - todo
>          in let look t =
>                  case t of
>                      Leaf h l | h == k ->
>                          apply_listd l d x
>                      Branch p b l r | (k `xor` p) .&. (b - 1) == 0 -> --  (Branch p b l r) | ((k xor p) .&. (b - 1)) == 0 ->
>                          look (if k .&. b == 0 then l else r)
>                      _ -> d x
>             in look f
>
>
>
> ===================
> Error:
> ===================
>
> Intro.hs:37:25: error:
>      * Ambiguous type variable `t0' arising from a use of `apply_listd'
>        prevents the constraint `(Ord t0)' from being solved.
>        Relevant bindings include
>          l :: [(t0, t)] (bound at Intro.hs:36:28)
>          t :: Func t0 t (bound at Intro.hs:34:21)
>          look :: Func t0 t -> t (bound at Intro.hs:34:16)
>          x :: t0 (bound at Intro.hs:32:15)
>          d :: t0 -> t (bound at Intro.hs:32:13)
>          f :: Func t0 t (bound at Intro.hs:32:11)
>          (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds)
>        Probable fix: use a type annotation to specify what `t0' should be.
>        These potential instances exist:
>          instance Ord Ordering -- Defined in `GHC.Classes'
>          instance Ord Integer
>            -- Defined in `integer-gmp-1.0.0.1:GHC.Integer.Type'
>          instance Ord a => Ord (Maybe a) -- Defined in `GHC.Base'
>          ...plus 22 others
>          ...plus five instances involving out-of-scope types
>          (use -fprint-potential-instances to see them all)
>      * In the expression: apply_listd l d x
>        In a case alternative: Leaf h l | h == k -> apply_listd l d x
>        In the expression:
>          case t of {
>            Leaf h l | h == k -> apply_listd l d x
>            Branch p b l r
>              | (k `xor` p) .&. (b - 1) == 0
>              -> look (if k .&. b == 0 then l else r)
>            _ -> d x }
>
>
> -- 
> Best wishes,
> Ivan Kush
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