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Today's Topics:
1. Re: Convert my own types to the Haskell types using
typeclass (David McBride)
2. Re: Convert my own types to the Haskell types using
typeclass (Magnus Therning)
3. Re: Convert my own types to the Haskell types using
typeclass (ke dou)
----------------------------------------------------------------------
Message: 1
Date: Fri, 11 Apr 2014 13:22:29 -0400
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Convert my own types to the Haskell
types using typeclass
Message-ID:
<CAN+Tr40soFtiQ2cmVTJv21_=T5haiPT=x=g=roxhwbz8anf...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
There is nothing to fix. It is doing exactly as you intended. You are
confusing types vs values. Integer is a type, 1, 2, and 3 are values.
toHaskell takes a type (MyOption a) and converts it to a type (Maybe a).
But in your actual code you will use values ie. toHaskell (Some 'a' ::
MyOption Char) and get back a (Just 'a' :: Maybe Char).
On Fri, Apr 11, 2014 at 1:13 PM, ke dou <[email protected]> wrote:
> Thank you, Bob.
> I also noticed that, but don't know how to fix it.
> Any suggestions?
>
> --Ke
>
>
> On Fri, Apr 11, 2014 at 1:05 PM, Bob Ippolito <[email protected]> wrote:
>
>> On Fri, Apr 11, 2014 at 9:50 AM, ke dou <[email protected]> wrote:
>>
>>> Hi,
>>>
>>> I want to define a typeclass that can convert my own types like MyBool,
>>> MyInt, MyOption to according Haskell types -- Bool, Int, Maybe.
>>>
>>> Currently I can convert the first two types, but for MyOption, I don't
>>> how to do, since it is a polymophic type.
>>>
>>> Here is my toy program:
>>> --------------------------------------------------------------
>>> {-# LANGUAGE TypeFamilies #-}
>>>
>>> module Coercible where
>>>
>>> import qualified Prelude
>>>
>>> data MyBool = MyTrue | MyFalse
>>> data MyInt = Zero | One | Two
>>> data MyOption a =
>>> Some a
>>> | None
>>>
>>> class Coercible a where
>>> type Return a :: *
>>> toHaskell :: a -> (Return a)
>>>
>>> instance Coercible MyBool where
>>> type Return MyBool = Prelude.Bool
>>> toHaskell MyTrue = Prelude.True
>>> toHaskell MyFalse = Prelude.False
>>>
>>> instance Coercible MyInt where
>>> type Return MyInt = Prelude.Int
>>> toHaskell Zero = 0
>>> toHaskell One = 1
>>> toHaskell Two = 2
>>>
>>> instance Coercible (MyOption a) where
>>> type Return (MyOption a) = Prelude.Maybe a
>>> toHaskell (Some a) = Prelude.Just a
>>> toHaskell None = Prelude.Nothing
>>>
>>> --------------------------------------------------------------------------
>>> The problem occurs when I am trying to use "toHaskell (Some MyBool)",
>>> the error message is "Not in scope: data constructor `MyBool'".
>>> Any hints will be appreciated !!
>>>
>>
>> `Some MyBool` isn't valid Haskell. MyBool is the name of a type, there's
>> no binding or constructor with that name. `toHaskell (Some MyTrue)`
>> evaluates to `Just MyTrue` as expected.
>>
>> _______________________________________________
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>
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Message: 2
Date: Fri, 11 Apr 2014 20:43:18 +0200
From: Magnus Therning <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Convert my own types to the Haskell
types using typeclass
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
On Fri, Apr 11, 2014 at 12:50:45PM -0400, ke dou wrote:
> instance Coercible (MyOption a) where
> type Return (MyOption a) = Prelude.Maybe a
> toHaskell (Some a) = Prelude.Just a
> toHaskell None = Prelude.Nothing
The basic issue has been solved in another thread, but wouldn't you
want this one to be something like this:
instance Coercible a => Coercible (MyOption a) where
type Return (MyOption a) = Prelude.Maybe (Return a)
toHaskell (Some a) = Prelude.Just (toHaskell a)
toHaskell None = Prelude.Nothing
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: [email protected] jabber: [email protected]
twitter: magthe http://therning.org/magnus
What gets measured, gets done.
-- Tom Peters
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Message: 3
Date: Fri, 11 Apr 2014 16:15:48 -0400
From: ke dou <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Convert my own types to the Haskell
types using typeclass
Message-ID:
<CAG1_UCTq3ek7=tTFR=1HonGtZifra=NFRH8u=nnw7g+hbol...@mail.gmail.com>
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Thank you very much!
That is exactly what I want.
Best,
Ke
On Fri, Apr 11, 2014 at 2:43 PM, Magnus Therning <[email protected]>wrote:
> On Fri, Apr 11, 2014 at 12:50:45PM -0400, ke dou wrote:
> > instance Coercible (MyOption a) where
> > type Return (MyOption a) = Prelude.Maybe a
> > toHaskell (Some a) = Prelude.Just a
> > toHaskell None = Prelude.Nothing
>
> The basic issue has been solved in another thread, but wouldn't you
> want this one to be something like this:
>
> instance Coercible a => Coercible (MyOption a) where
> type Return (MyOption a) = Prelude.Maybe (Return a)
> toHaskell (Some a) = Prelude.Just (toHaskell a)
> toHaskell None = Prelude.Nothing
>
> /M
>
> --
> Magnus Therning OpenPGP: 0xAB4DFBA4
> email: [email protected] jabber: [email protected]
> twitter: magthe http://therning.org/magnus
>
> What gets measured, gets done.
> -- Tom Peters
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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