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Today's Topics:
1. Re: Pattern match(es) are overlapped ... but I do not see
that they do (Michael Orlitzky)
2. Re: Pattern match(es) are overlapped ... but I do not see
that they do (Nathan H?sken)
----------------------------------------------------------------------
Message: 1
Date: Sat, 31 Aug 2013 23:39:22 -0400
From: Michael Orlitzky <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Pattern match(es) are overlapped ...
but I do not see that they do
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
On 08/31/2013 11:07 PM, Edward Z. Yang wrote:
> Well, the trouble is at the source language level there
> is no way to tell if some_var is /actually/ a constant, or
> some complicated expression. So you could do guards:
>
> case res of
> _ | res == wxID_CANCEL -> True
> | res == wxID_NO -> False
> | ...
>
> The suggestion to use an ADT is, you write a helper fucntion
> which does this case-split first, and then you do regular pattern
> matching on the result. If you need to do this multiple times,
> it saves you a bunch of typing; it also gives you one place
> to write the error code when the integer is not one of these
> three values.
>
In a perfect world, these constants would be defined as part of an
enumeration type, correct? For example,
data WxId = WxIdCancel | WxIdNo | WxIdYes ... deriving (Enum)
in which case the original attempt would have succeeded since it would
be matching on a constructor.
However, unless there are constants defined for 0,1,... this approach
won't work automatically -- the derived Enum instance starts at zero and
increments by one. The library would have to define a custom Enum
instance and it would add a good bit of code.
------------------------------
Message: 2
Date: Sun, 01 Sep 2013 09:36:36 +0200
From: Nathan H?sken <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Pattern match(es) are overlapped ...
but I do not see that they do
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey,
I ended up using guards. If I end up needing this at multiple locations,
I will introduce an ADT.
This was very helpful!
Regards,
Nathan
On 09/01/2013 05:39 AM, Michael Orlitzky wrote:
> On 08/31/2013 11:07 PM, Edward Z. Yang wrote:
>> Well, the trouble is at the source language level there
>> is no way to tell if some_var is /actually/ a constant, or
>> some complicated expression. So you could do guards:
>>
>> case res of
>> _ | res == wxID_CANCEL -> True
>> | res == wxID_NO -> False
>> | ...
>>
>> The suggestion to use an ADT is, you write a helper fucntion
>> which does this case-split first, and then you do regular pattern
>> matching on the result. If you need to do this multiple times,
>> it saves you a bunch of typing; it also gives you one place
>> to write the error code when the integer is not one of these
>> three values.
>>
> In a perfect world, these constants would be defined as part of an
> enumeration type, correct? For example,
>
> data WxId = WxIdCancel | WxIdNo | WxIdYes ... deriving (Enum)
>
> in which case the original attempt would have succeeded since it would
> be matching on a constructor.
>
> However, unless there are constants defined for 0,1,... this approach
> won't work automatically -- the derived Enum instance starts at zero and
> increments by one. The library would have to define a custom Enum
> instance and it would add a good bit of code.
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
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