Yes, if you use a bang pattern in a let clause it can act as an assert in a
strict context (e.g. in IO Monad). But, it can't even be caught as an
exception (or can it?). For example, this fails:
x = fromList [("x", 1)] :: Map String Int
main = print $ let !Nothing = Data.Map.lookup "x" x in 10
x: x.hs:22:20-51: Irrefutable pattern failed for pattern Nothing
-harendra
On 24 February 2017 at 09:21, Brandon Allbery
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
On 24 February 2017 at 08:45, Brandon Allbery
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
wrote: pattern match be ever actually tried when there is no binding?
-harendra
-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net