[Haskell-beginners] type signature error in a where clause
Mark Wallace
lotabout at gmail.com
Sun Nov 25 16:31:13 CET 2012
> The unspoken wisdom goes something like this: the classic top-down FP way
> of coding has you sketch out most if not all of your function signatures.
> So when you hit the keyboard, a very natural thing to do is to key in all
> those signatures stubbing out definitions using "undefined". You proceed
> from there.
>
> Sometimes people just use haskell as a calculator on steroids, especially
> when solving project-euler-type problems. In which case, anything goes.
> Needless to say, if all the practice a beginner gets is project euler,
> they're missing out a lot.
Thanks very much for the tips and thought.
Nowadays I'm working with the Haskell 99 Questions, and my next target
would be Project Euler. Someone have suggested to implement Prelude by
myself. But still can't find the 'right' thing to do.
Would you mind sharing with us your experience of learning haskell? What
would you recommend or what have you done in order to improve?
On 11/25/2012 05:34 PM, Kim-Ee Yeoh wrote:
> On Sat, Nov 24, 2012 at 10:32 PM, Mark Wallace <lotabout at gmail.com> wrote:
>
>>> Somehow it might seem a bit easier to me to grasp the function of a
> function with the help of type signature. I'll try just omitting the
> signatures, it's easier and more handy isn't it?
>
> The unspoken wisdom goes something like this: the classic top-down FP way
> of coding has you sketch out most if not all of your function signatures.
> So when you hit the keyboard, a very natural thing to do is to key in all
> those signatures stubbing out definitions using "undefined". You proceed
> from there.
>
> Sometimes people just use haskell as a calculator on steroids, especially
> when solving project-euler-type problems. In which case, anything goes.
> Needless to say, if all the practice a beginner gets is project euler,
> they're missing out a lot.
>
>
> -- Kim-Ee
>
>
> On Sat, Nov 24, 2012 at 10:32 PM, Mark Wallace <lotabout at gmail.com> wrote:
>
>> On 11/24/2012 11:07 PM, Daniel Fischer wrote:
>>
>>> On Samstag, 24. November 2012, 22:04:15, Mark Wallace wrote:
>>>
>>>> I'm writing a merge sort function, but I get type error under such
>>>> implementation:
>>>>
>>>> mergesort :: (a -> a -> Ordering) -> [a] -> [a]
>>>> mergesort cmp xs = mergeAll (map (\x -> [x]) xs)
>>>> where
>>>> mergeAll :: [[a]] -> [a]
>>>> mergeAll [x] = x
>>>> mergeAll xs = mergeAll (mergePairs xs)
>>>>
>>>> mergePairs :: [[a]] -> [[a]]
>>>> mergePairs (a:b:xs) = merge a b : mergePairs xs
>>>> mergePairs xs = xs
>>>>
>>>> merge :: [a] -> [a] -> [a]
>>>> merge as@(a:as') bs@(b:bs')
>>>>
>>>> | cmp a b == GT = b : merge as bs'
>>>> | otherwise = a : merge as' bs
>>>>
>>>> merge [] bs = bs
>>>> merge as [] = as
>>>>
>>>> And ghc says:
>>>>
>>>> Couldn't match type `a1' with `a'
>>>> `a1' is a rigid type variable bound by
>>>> the type signature for merge :: [a1] -> [a1] -> [a1]
>>>> at
>>>> /home/ice/Study/Haskell/**tutorials/99Questions/21to30.**hs:135:7
>>>> `a' is a rigid type variable bound by
>>>> the type signature for
>>>> mergesort :: (a -> a -> Ordering) -> [a] -> [a]
>>>> at
>>>> /home/ice/Study/Haskell/**tutorials/99Questions/21to30.**hs:124:1
>>>> In the first argument of `cmp', namely `a'
>>>> In the first argument of `(==)', namely `cmp a b'
>>>> In the expression: cmp a b == GT
>>>>
>>>> But if I comment all type signatures, ghc works fine on it.
>>>> I would really appreciate it if you can point out what causes this
>>>> question?
>>>>
>>> Type variables are implicitly for all-quantified. Thus the type variable
>>> a in
>>> the signatures of the local functions is a fresh type variable and has
>>> nothing
>>> to do with the a from the top-level signature.
>>>
>>> It is equivalent to you writing
>>>
>>> merge :: [b] -> [b] -> [b]
>>>
>>> except there it is obvious that the type signature is wrong.
>>>
>>> And how to fix it without changing the structure of the program (i.e. not
>>> adding function `cmp' as a parameter of `merge' etc.).
>>>
>>> 1. Just omit the type signatures, they can be inferred.
>>>
>>> That's the portable way.
>>>
>>> 2. Bring the type variable a into scope
>>>
>>> {-# LANGUAGE ScopedTypeVariables #-}
>>>
>>> mergesort :: forall a. (a-> a-> Ordering) -> [a] -> [a]
>>>
>>> then an (unquantified) a in a local type signature refers to the type
>>> from the
>>> top-level signature.
>>>
>>> That's a GHC-only (as far as I know) way.
>>>
>> Thanks for answering so fast. And all of your answers are very helpful.
>> I've tested these two solutions, all works fine.
>> Now I understand how type signature works in such condition.
>>
>> Somehow it might seem a bit easier to me to grasp the function of a
>> function with the help of type signature.
>> I'll try just omitting the signatures, it's easier and more handy isn't it?
>>
>>
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