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

Brandon Allbery allbery.b at gmail.com
Fri Feb 24 03:51:21 UTC 2017

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>

> 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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