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Today's Topics:
1. Re: Type declarations (Francesco Ariis)
2. Re: Type declarations (Patrik Iselind)
3. Re: Type declarations (Francesco Ariis)
4. Re: Why do i need to specify the class of a here at all?
(Patrik Iselind)
5. Re: Type declarations (Patrik Iselind)
6. Re: Type declarations (Francesco Ariis)
7. Re: Type declarations (mrx)
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Message: 1
Date: Sun, 26 Nov 2017 21:24:01 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Type declarations
Message-ID: <20171126202401.jppfe6tmm67lf...@x60s.casa>
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On Sun, Nov 26, 2017 at 07:39:49PM +0100, Patrik Iselind wrote:
> Does this mean that i can write `delta :: Point Double t -> Point Double t
> -> Direction d` as a type declaration. Then i would require `Coordinate
> Double Double` as in parameters. Correct?
Careful there! Let's take a simple data declaration
data Point = Point Int Float
On the right you have *the type*, on the left you have *the (data)
constructor*.
When you are writing signatures you are writing types, so
f :: Point -> Point
and not
f :: Point Int Float -> Point Int Float
Much like you write `addition :: Int -> Int -> Int` and not
`addition :: 7 -> 2 -> 9`.
------------------------------
Message: 2
Date: Sun, 26 Nov 2017 22:21:39 +0100
From: Patrik Iselind <patrik....@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Type declarations
Message-ID: <d6445383-2ad4-6d53-5d02-bc94e0616...@gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Den 2017-11-26 kl. 21:24, skrev Francesco Ariis:
> On Sun, Nov 26, 2017 at 07:39:49PM +0100, Patrik Iselind wrote:
>> Does this mean that i can write `delta :: Point Double t -> Point Double t
>> -> Direction d` as a type declaration. Then i would require `Coordinate
>> Double Double` as in parameters. Correct?
> Careful there! Let's take a simple data declaration
>
> data Point = Point Int Float
>
> On the right you have *the type*, on the left you have *the (data)
> constructor*.
But if the type Point have a parameter, `data Point p = Coordinate p p`.
Then i must be able to tell which `p` when i use Point in a type
declaration right? Like `delta :: Point Double a -> Point Double b ->
Double` would say that p is Double. How else am i supposed to specify p
in type declaration?
// Patrik
------------------------------
Message: 3
Date: Sun, 26 Nov 2017 22:41:35 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Type declarations
Message-ID: <20171126214135.732pblhx4ouym...@x60s.casa>
Content-Type: text/plain; charset=us-ascii
On Sun, Nov 26, 2017 at 10:21:39PM +0100, Patrik Iselind wrote:
> But if the type Point have a parameter, `data Point p = Coordinate p p`.
> Then i must be able to tell which `p` when i use Point in a type declaration
> right? Like `delta :: Point Double a -> Point Double b -> Double` would say
> that p is Double. How else am i supposed to specify p in type declaration?
If the type has a parameter, like
data Point a = Coordinates a a
it will specified *once* in the signature, like this:
f :: Point Double -> Point Double -> String
implementation looking something like
f (Coordinates x y) (Coordinates m q) = x + 7 -- etc. etc.
------------------------------
Message: 4
Date: Sun, 26 Nov 2017 22:46:29 +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: <3baa5db0-8901-0029-1366-eb06e2359...@gmail.com>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
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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Message: 5
Date: Sun, 26 Nov 2017 22:51:26 +0100
From: Patrik Iselind <patrik....@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Type declarations
Message-ID: <fb39bc47-01fe-e366-1bb2-6b5fd2fb3...@gmail.com>
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Patrik Iselind
Den 2017-11-26 kl. 22:41, skrev Francesco Ariis:
> On Sun, Nov 26, 2017 at 10:21:39PM +0100, Patrik Iselind wrote:
>> But if the type Point have a parameter, `data Point p = Coordinate p p`.
>> Then i must be able to tell which `p` when i use Point in a type declaration
>> right? Like `delta :: Point Double a -> Point Double b -> Double` would say
>> that p is Double. How else am i supposed to specify p in type declaration?
> If the type has a parameter, like
>
> data Point a = Coordinates a a
>
> it will specified *once* in the signature, like this:
>
> f :: Point Double -> Point Double -> String
So what you're saying is that the `a` and `b` characters in my examples
should not be there. Other than that it's correct?
// Patrik
------------------------------
Message: 6
Date: Sun, 26 Nov 2017 23:31:05 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Type declarations
Message-ID: <20171126223104.btdjdyinmdvgp...@x60s.casa>
Content-Type: text/plain; charset=us-ascii
On Sun, Nov 26, 2017 at 10:51:26PM +0100, Patrik Iselind wrote:
> So what you're saying is that the `a` and `b` characters in my examples
> should not be there. Other than that it's correct?
Indeed :)
------------------------------
Message: 7
Date: Mon, 27 Nov 2017 09:01:19 +0100
From: mrx <patrik....@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Type declarations
Message-ID:
<CANzOjBhDVfmyZDbyM=0cx3GHUS2-ZUTCRy1k0Nmqu8b8BXFC=g...@mail.gmail.com>
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Den 26 nov 2017 23:32 skrev "Francesco Ariis" <fa...@ariis.it>:
On Sun, Nov 26, 2017 at 10:51:26PM +0100, Patrik Iselind wrote:
> So what you're saying is that the `a` and `b` characters in my examples
> should not be there. Other than that it's correct?
Indeed :)
Awesome, thanks a lot!
// Patrik
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