Oh golly. I can't live without pattern guards. they are the best darn
thing since sliced bread. I highly recommend them. I .. would have
to seriously refactor a ton of code if pattern guards disapeared.
So much so that it would be less work to implement pattern guards for
other compilers and submit the patches.
eh, well, pattern guards are nice. but, many of their uses are also
easily replaced, by moving the pg-monad from the lhs to the rhs,
and employing MonadPlus for the fall-throughs; something roughly
like:
f p1 | g1 = e1
..
f pn | gn = en
where <stuff>
-->
f x = fromJust $ do {p1 <- return x; guard g1; return e1}
..
`mplus` do {pn <- return x; guard gn; return en}
where <stuff>
for any pattern matches in the guards, just drop the "guard";-)
grouping several guards under one pattern is also straightforward.
the only problem is where to place any pattern-dependent wheres.
the easy approach matches them lazily out of x, but that matches
some things twice..
this is not an argument against pattern guards; but I often find to
my surprise that many Haskellers still think that functions with
several clauses can't be translated easily (without losing the
basic structure [*]) because of the "fall-through" on pattern-match
failure, so they might find this translation pattern useful;
it could also be elaborated (recovering better error messages)
into a naive, but readable, desugaring of pattern-guards and the
like, for reports, and for not-yet-supporting implementations..
cheers,
claus
[*] to be more accurate, since you can move the whole group
of clauses into a case, the barrier is seen between moving
from the world of patterns and guards to the world of
expressions. MonadPlus allows us to move back.
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