optimization question

[Simon Peyton-Jones]

The trouble is that you probably *don't* want to expand this
	case x of { "foogle" -> e1; _ -> e2 }
to this

  case x of
     c1:x1 -> case c1 of
		'f' -> case x1 of
		  	c2:x2 -> case c2 of 
					'o' -> of ....

So GHC generates a series of equality tests instead.  A decent
alternative might be:
	
	generate case expressions when there is more
	than one string in the list, otherwise use an equality test

That would not be hard to do.  If it becomes important to you, I'd have
a go. But before doing so, could you do the work by hand and see if it
makes a useful performance difference?

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Sven Panne
| Sent: 22 February 2004 15:32
| To: John Meacham
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: optimization question
| 
| John Meacham wrote:
| > I was wondering if:
| >
| > case x of
| >         "foo" -> Foo
| >         "bar" -> Bar
| >         "fuzz" -> Fuzz
| >         "fuzo" -> Fuzo
| >         x -> other .. thing
| >
| > would optimize to
| >
| > let z = other .. thing in
| > case x of
| >         ('f':x) -> case x of
| >                 ('u':'z': x) ->
| >                         "z" -> Fuzz
| >                         "o" -> Fuzo
| >                         _ -> z
| >                 "oo" -> Foo
| >                 _ -> z
| >         "bar" -> Bar
| >         _ -> z
| 
| String literals are handled in a special way in GHC, so your example
is
| essentially converted into an if-cascade, which is not what you want.
| OTOH, if you write the strings in their expanded form like
['f','o','o'],
| you get your optimized version automatically. Perhaps Simon^2 can
comment
| on the rationale behind this, I can't remember the reason...
| 
| Cheers,
|     S.
| 
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