#4359: Implement lambda-case/lambda-if
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner: igloo
Type: feature request | Status: patch
Priority: high | Milestone: 7.4.1
Component: Compiler | Version: 7.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
Comment(by mikhail.vorozhtsov):
Replying to [comment:48 simonpj]:
> Thus:
> {{{
> \ { True -> e1; False -> e2 }
> }}}
> would be ok. But this would not be ok
> {{{
> \ True -> e1
> False -> e2
> }}}
> That seems like a reasonable compromise. Using the brace/semicolon
stuff is compatible with the way case expressions work, which is a Good
Thing. Any comments?
Ugh. This is getting uglier and uglier. I've been using the current
version of the patch for some time and I think the syntax has two major
problems:
1. It rejects the nice-looking indentation we are all used to in the
single-clause case (see my 'mask' example above).
2. It looks like a case expression, but has different pattern syntax, i.e.
I constantly write
{{{
foo >>= \
Nothing -> ...
Just a -> ...
}}}
instead of the correct
{{{
foo >>= \
Nothing -> ...
(Just a) -> ...
}}}
Explicit layout solves the first issue, IMO, at the expense of graphic
clarity. I mean
{{{
foo >>= \
{ Nothing -> do
abc
def
; (Just a) -> do
abc
def
}
}}}
just makes my eyes hurt. Do we really want a feature that is only
accessible using explicit layout?
I think the fact that single- and multi-clause lambdas require different
indentation rules (and we can't look ahead to distinguish them) tells us
to use a different token (\fn, \fun, \let?). I'd also say that {{{\x ->
case x of ...}}} (or {{{x <- foo; case x of ...}}}) is such a common idiom
that it deserves its "anonymous" counterpart (with respect to pattern
syntax) regardless of the whole multi-clause lambda thing.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4359#comment:49>
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