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Today's Topics:
1. Custom type classes (Daniel Hinojosa)
2. Re: Custom type classes (Daniel Hinojosa)
3. Re: Custom type classes (Daniel Hinojosa)
4. Re: Custom type classes (Imants Cekusins)
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Message: 1
Date: Sun, 24 Jan 2016 23:25:49 -0700
From: Daniel Hinojosa <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Custom type classes
Message-ID:
<caoxuh-4pgfbqpso1gzthjxdfpgsx_vdiccp+en5xbmg0fgq...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I am pretty sure I have a good handle on type classes, but this one is
perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3
(I know Haskell already has Tuples, just thought I would create my own for
this exercise). Then I wanted to create a type class that would have a
method called first that would get the first element of the tuple
regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.
Here is what I have:
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx where
first :: idx c -> a
instance Indexable (Tuple2 a) where
first (Tuple2 a b) = a
In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"
I was greeted with the following trace:
Couldn't match expected type ?a1? with actual type ?a?
?a? is a rigid type variable bound by
the instance declaration at TypeClasses.hs:35:10
?a1? is a rigid type variable bound by
the type signature for first :: Tuple2 a c -> a1
at TypeClasses.hs:36:4
Relevant bindings include
a :: a (bound at TypeClasses.hs:36:18)
first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4)
In the expression: a
In an equation for ?first?: first (Tuple2 a b) = a
Help is appreciated.
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Message: 2
Date: Sun, 24 Jan 2016 23:39:11 -0700
From: Daniel Hinojosa <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Custom type classes
Message-ID:
<caoxuh-49cf+8bofkiniexy3jdpyzgms4znwdskckhv5wxz-...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Quick adjustment, playing around too much with it, that should be:
class Indexable idx where
first :: idx a -> a
Problem still exists.
On Sun, Jan 24, 2016 at 11:25 PM, Daniel Hinojosa <
[email protected]> wrote:
> I am pretty sure I have a good handle on type classes, but this one is
> perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3
> (I know Haskell already has Tuples, just thought I would create my own for
> this exercise). Then I wanted to create a type class that would have a
> method called first that would get the first element of the tuple
> regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.
>
> Here is what I have:
>
> data Tuple3 a b c = Tuple3 a b c deriving (Show)
>
> data Tuple2 a b = Tuple2 a b deriving (Show)
>
> class Indexable idx where
> first :: idx c -> a
>
> instance Indexable (Tuple2 a) where
> first (Tuple2 a b) = a
>
> In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"
>
> I was greeted with the following trace:
> Couldn't match expected type ?a1? with actual type ?a?
> ?a? is a rigid type variable bound by
> the instance declaration at TypeClasses.hs:35:10
> ?a1? is a rigid type variable bound by
> the type signature for first :: Tuple2 a c -> a1
> at TypeClasses.hs:36:4
> Relevant bindings include
> a :: a (bound at TypeClasses.hs:36:18)
> first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4)
> In the expression: a
> In an equation for ?first?: first (Tuple2 a b) = a
>
> Help is appreciated.
>
>
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Message: 3
Date: Sun, 24 Jan 2016 23:43:23 -0700
From: Daniel Hinojosa <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Custom type classes
Message-ID:
<CAOXUh-6STv0j-oBk+uAYOg0EEfxgrKB_9y92hcnN35fFNLf=_...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Got it to work this way but it got the wrong one. Still looking.
instance Indexable (Tuple2 a) where
first (Tuple2 b a) = a
On Sun, Jan 24, 2016 at 11:39 PM, Daniel Hinojosa <
[email protected]> wrote:
> Quick adjustment, playing around too much with it, that should be:
>
> class Indexable idx where
> first :: idx a -> a
>
> Problem still exists.
>
> On Sun, Jan 24, 2016 at 11:25 PM, Daniel Hinojosa <
> [email protected]> wrote:
>
>> I am pretty sure I have a good handle on type classes, but this one is
>> perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3
>> (I know Haskell already has Tuples, just thought I would create my own for
>> this exercise). Then I wanted to create a type class that would have a
>> method called first that would get the first element of the tuple
>> regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.
>>
>> Here is what I have:
>>
>> data Tuple3 a b c = Tuple3 a b c deriving (Show)
>>
>> data Tuple2 a b = Tuple2 a b deriving (Show)
>>
>> class Indexable idx where
>> first :: idx c -> a
>>
>> instance Indexable (Tuple2 a) where
>> first (Tuple2 a b) = a
>>
>> In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"
>>
>> I was greeted with the following trace:
>> Couldn't match expected type ?a1? with actual type ?a?
>> ?a? is a rigid type variable bound by
>> the instance declaration at TypeClasses.hs:35:10
>> ?a1? is a rigid type variable bound by
>> the type signature for first :: Tuple2 a c -> a1
>> at TypeClasses.hs:36:4
>> Relevant bindings include
>> a :: a (bound at TypeClasses.hs:36:18)
>> first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4)
>> In the expression: a
>> In an equation for ?first?: first (Tuple2 a b) = a
>>
>> Help is appreciated.
>>
>>
>
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Message: 4
Date: Mon, 25 Jan 2016 08:42:10 +0100
From: Imants Cekusins <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Custom type classes
Message-ID:
<cap1qinzxkabm2-w25kjd-z6athvu8twzgadz2jkr9vwsqyz...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Hello Daniel,
it works with these tweaks:
-- begin
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}
module TupInst where
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx a where
first :: idx -> a
instance Indexable (Tuple2 a b) a where
first (Tuple2 a0 b0) = a0
instance Indexable (Tuple3 a b c) a where
first (Tuple3 a0 b0 c0) = a0
-- end
call it in ghci like this:
first $ Tuple3 (1::Int) 'a' False::Int
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