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

Taeer Bar-Yam taeer at necsi.edu
Fri Feb 24 05:00:33 UTC 2017

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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