[Haskell-beginners] Why do i need to specify the class of a here at all?

[Patrik Iselind]

Den 2017-11-26 kl. 20:48, skrev Quentin Liu:
>> ```
>> exercises.hs:33:13:
>>     Couldn't match expected type ‘[b0]’ with actual type ‘a’
>>       ‘a’ is a rigid type variable bound by
>>           the type signature for myOrderFunc :: a -> a -> Ordering
>>           at exercises.hs:31:16
>>     Relevant bindings include
>>       y :: a (bound at exercises.hs:32:15)
>>       x :: a (bound at exercises.hs:32:13)
>>       myOrderFunc :: a -> a -> Ordering (bound at exercises.hs:32:1)
>>     In the first argument of ‘myLen’, namely ‘x’
>>     In the first argument of ‘(<)’, namely ‘myLen x’
>> Failed, modules loaded: none.
>> ```
>
> Your guess is correct. The problem is, Haskell does not consider `a` 
> in `myOrderFunc` and `[b]` in `myLen` equivalent. `a` means you feed 
> the function any type, while `[b]` means it must be a list of values 
> of the same type. So changing `a` to `[a]` woud eliminate the error.
Thanks a lot for the clarification.

// Patrik
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