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

Ivan Kush ivan.v.kush at yandex.ru
Sat Jan 28 20:09:09 UTC 2017

```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
```