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Today's Topics:
1. Re: Why do i need to specify the class of a here at all?
(Patrik Iselind)
2. Fwd: Re: Multiple letters between -> -> (Marcus Manning)
----------------------------------------------------------------------
Message: 1
Date: Fri, 24 Nov 2017 22:32:17 +0100
From: Patrik Iselind <patrik....@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Why do i need to specify the class of
a here at all?
Message-ID: <0b94896c-7d1f-bd65-2177-3b0f2d54a...@gmail.com>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Den 2017-11-24 kl. 20:04, skrev Quentin Liu:
>
>> Yes, you could pass the function a list of strings as well. A
>> string is just a list of Chars. The type signature `a` does not
>> restrict the range of types you could pass to the function.
>>
>> That seem strange to me. Wouldn't that mean that i could write the
>> declaration of myOrderFunc as `myOrderFunc :: a -> a -> Ordering` as
>> well? GHCI give me an error on this though so obviously it's wrong. I
>> just don't see why. Why cannot a represent [b]?
>
> Could you copy and paste the error message here?
Sure, the error i get follows
```
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.
```
Attaching the updated exercises.hs for reference.
I'm still not very good at interpreting Haskell's error messages, they
are quite cryptic to me. My interpretation/guess of the above is that my
`a` is too 'wide' or how you express it. Haskell seem to expect some
form of list. Most likely since i want a length and lists are perhaps
everything in Haskell that can produce a length. I've hardly scratched
the surface of what i imagine is Haskell so i cannot say anything for
sure yet.
>
> The way I use to think about type signature is, when you trying to
> substitute type variables such as `a`, substitute it into a concrete
> type that you are working with.
I'm having a hard time understanding your way of thinking about type
signatures. Could you perhaps elaborate a bit more on it?
// Patrik
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Message: 2
Date: Sat, 25 Nov 2017 13:06:03 +0100
From: Marcus Manning <icons...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Fwd: Re: Multiple letters between -> ->
Message-ID: <ef75b987-dee3-ff03-2e1f-f1a28d833...@gmail.com>
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-------- Forwarded Message --------
Subject: Re: [Haskell-beginners] Multiple letters between -> ->
Date: Sat, 25 Nov 2017 13:05:25 +0100
From: Marcus Manning <icons...@gmail.com>
To: Francesco Ariis <fa...@ariis.it>
Sorry, for the long break.
Thanks for replying.
I do not believe that h is a higher kinded type. What I want to express
is that a function f could take a type constructor as argument and
simply returns it, but
f Maybe
throws an Error
<interactive>:13:3: error:
• Data constructor not in scope: Maybe :: h a
• Perhaps you meant variable ‘maybe’ (imported from Prelude)
So what instead does h a mean in a function declaration?
Cheers,
Marcus.
On 11/23/2017 06:27 PM, Francesco Ariis wrote:
> On Thu, Nov 23, 2017 at 06:19:51PM +0100, Marcus Manning wrote:
>> Hi,
>>
>> Original I thought a Signature like:
>>
>> f :: h a -> h a
>>
>> means that h is a higher kinded type just like in Type Classes ( for
>> instance f in Functor f).
>>
>> But I heard such a meaning is not allowed in normal Haskell functions. What
>> instead is the meaning of h a?
> Hello Marcus,
> you can write that but, since we know nothing about `h` and `a`,
> the only possible (non-undefined) function to implement that
> signature is:
>
> f :: h a -> h a
> f = id
>
> Any other implementation would require us to know something about h,
> hence a typeclass-constraint (e.g. Functor h =>) on h.
> _______________________________________________
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