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Today's Topics:
1. Using symbolic values in the case expression (C K Kashyap)
2. Re: Using symbolic values in the case expression (Thomas Davie)
3. Re: Using symbolic values in the case expression (Daniel Fischer)
4. Re: Using symbolic values in the case expression (C K Kashyap)
5. Re: Using symbolic values in the case expression (Ozgur Akgun)
6. Re: Using symbolic values in the case expression (Daniel Fischer)
----------------------------------------------------------------------
Message: 1
Date: Thu, 11 Nov 2010 22:33:10 +0530
From: C K Kashyap <ckkash...@gmail.com>
Subject: [Haskell-beginners] Using symbolic values in the case
expression
To: beginners@haskell.org
Message-ID:
<aanlktime3nmfbhocozgxbr5p22xys9hvbsu_hndsc...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hi,
I'd like to do something like this -
Instead of doing this -
f x = case x of
1 -> "One"
2 -> "Two"
I'd like to do this -
v1 = 1
v2 = 2
f x = case of
v1 -> "One"
v2 -> "Two"
Is that possible?
--
Regards,
Kashyap
------------------------------
Message: 2
Date: Thu, 11 Nov 2010 17:06:06 +0000
From: Thomas Davie <tom.da...@gmail.com>
Subject: Re: [Haskell-beginners] Using symbolic values in the case
expression
To: C K Kashyap <ckkash...@gmail.com>
Cc: beginners@haskell.org
Message-ID: <bedde69f-d171-44ee-9464-0c97b356a...@gmail.com>
Content-Type: text/plain; charset=us-ascii
On 11 Nov 2010, at 17:03, C K Kashyap wrote:
> Hi,
> I'd like to do something like this -
>
> Instead of doing this -
>
> f x = case x of
> 1 -> "One"
> 2 -> "Two"
>
> I'd like to do this -
>
> v1 = 1
> v2 = 2
>
> f x = case of
> v1 -> "One"
> v2 -> "Two"
>
>
> Is that possible?
Pattern matching against expressions is not possible, except in the very very
limited case of n + k patterns, so no.
Bob
------------------------------
Message: 3
Date: Thu, 11 Nov 2010 18:24:34 +0100
From: Daniel Fischer <daniel.is.fisc...@web.de>
Subject: Re: [Haskell-beginners] Using symbolic values in the case
expression
To: beginners@haskell.org
Message-ID: <201011111824.34671.daniel.is.fisc...@web.de>
Content-Type: text/plain; charset="iso-8859-1"
On Thursday 11 November 2010 18:03:10, C K Kashyap wrote:
> Hi,
> I'd like to do something like this -
>
> Instead of doing this -
>
> f x = case x of
> 1 -> "One"
> 2 -> "Two"
>
> I'd like to do this -
>
> v1 = 1
> v2 = 2
>
> f x = case of
> v1 -> "One"
> v2 -> "Two"
>
>
> Is that possible?
No, case does a pattern match, it checks whether the expression matches the
pattern (patterns are defined in the report, section 3.17,
http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17),
so variable names (identifiers beginning with a lower case letter) match
everything.
------------------------------
Message: 4
Date: Thu, 11 Nov 2010 23:15:43 +0530
From: C K Kashyap <ckkash...@gmail.com>
Subject: Re: [Haskell-beginners] Using symbolic values in the case
expression
To: Daniel Fischer <daniel.is.fisc...@web.de>
Cc: beginners@haskell.org
Message-ID:
<aanlktinqebsabqxzqnfi7armqz2sugvvww=gapqr9...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Is there a way out?
What I need to do is implement a protocol - so it would be nice to
write a case expression with patters being symbolic.
--
Regards,
Kashyap
------------------------------
Message: 5
Date: Thu, 11 Nov 2010 18:42:32 +0000
From: Ozgur Akgun <ozgurak...@gmail.com>
Subject: Re: [Haskell-beginners] Using symbolic values in the case
expression
To: C K Kashyap <ckkash...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<aanlktinjrptq5z_rsbjdsvjvkc5xu6hczkdg93drh...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On 11 November 2010 17:03, C K Kashyap <ckkash...@gmail.com> wrote:
> v1 = 1
> v2 = 2
>
> f x = case x of
> v1 -> "One"
> v2 -> "Two"
>
The closest I can thin is the following:
f x | x == v1 = "One"
| x == v2 = "Two"
But, keep in mind, this is *not* the same thing. It requires Eq on x, and
you lose all the exhaustiveness/overlapping checks pattern matching
provides.
HTH,
--
Ozgur Akgun
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Message: 6
Date: Thu, 11 Nov 2010 19:18:33 +0100
From: Daniel Fischer <daniel.is.fisc...@web.de>
Subject: Re: [Haskell-beginners] Using symbolic values in the case
expression
To: C K Kashyap <ckkash...@gmail.com>
Cc: beginners@haskell.org
Message-ID: <201011111918.33536.daniel.is.fisc...@web.de>
Content-Type: text/plain; charset="iso-8859-1"
On Thursday 11 November 2010 18:45:43, C K Kashyap wrote:
> Is there a way out?
> What I need to do is implement a protocol - so it would be nice to
> write a case expression with patters being symbolic.
Guards?
case expr of
res | res == v1 -> "One"
| res == v2 -> "Two"
_ -> error "Three"
Or perhaps a little preprocessor abuse might help.
{-# LANGUAGE CPP #-}
#define V1 1
#define V2 2
case expr of
V1 -> "One"
V2 -> "Two"
_ -> error "Three"
------------------------------
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