[Haskell-cafe] What does "1 = 2" mean in Haskell?

[Harendra Kumar]

Pattern match in a function definition is different. Here the pattern match
is actually being used as part of the function definition.  We were trying
to figure out the usefulness of non-function pattern matches which do not
result in a binding. The assert case pointed out by Brandon is one example.
Though not so useful.

-harendra

On 24 February 2017 at 10:30, Taeer Bar-Yam <taeer at necsi.edu> wrote:

> Since nobody has provided an example use case, I will. It's not
> particularly useful, but it's a minimal case that does something
> interesting.
>
> func :: (Int, a) -> Maybe a
> func (0, x) = Just x
> func _ = Nothing
>
> Excerpts from Brandon Allbery's message of February 23, 2017 10:51 pm:
>
>> Without a binding it is useless at top level, but if you strictify the
>> pattern it can be useful in `let` (possibly as a sanity check where you
>> want the program to abort if it fails). I don't recall offhand if it
>> desugars usefully in list comprehensions, but if so it would work as a
>> filter. There may also be other specialized use cases; general syntax
>> tends
>> to get reused a lot in Haskell, so making this case a syntax error could
>> make it difficult to support actually useful cases. :)
>>
>> (Also I'm sure someone overly clever could figure out some way to abuse
>> it.
>> :)
>>
>> On Thu, Feb 23, 2017 at 10:41 PM, Harendra Kumar <
>> harendra.kumar at gmail.com>
>> wrote:
>>
>> On 24 February 2017 at 08:45, Brandon Allbery <allbery.b at gmail.com>
>>> wrote:
>>>
>>> Literally the only use I've seen for this was a CCC puzzle. However, it
>>>> is the trivial case of something that is more useful: pattern matching
>>>> the
>>>> result of an expression (say, a Data.Map.lookup when you know the key
>>>> exists).
>>>>
>>>>
>>> Can you explain how that will be useful (without a binding)? Will the
>>> pattern match be ever actually tried when there is no binding?
>>>
>>> -harendra
>>>
>>>
>>
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
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>
>  --Taeer
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