[Haskell-cafe] Call for discussion: OverloadedLists extension

Simon Peyton-Jones simonpj at microsoft.com
Tue Sep 25 10:01:06 CEST 2012

| Here at the University of Tübingen, I am co-supervising (together with
| Jeroen Weijers) a student project implementing the OverloadedLists
| extension for GHC. Achim Krause is the student who is working on the
| project. We took into consideration earlier discussions on this topic
| [1,2] before embarking on the project.
| Achim has worked on two approaches.

Your second approach is this:

| [x,y,z] 
| as
| singleton x `mappend` singleton y `mappend` singleton z ;

This approach is not good for long literal lists, because you get tons of executable code where the user thought he was just defining a data  structure.  And long literal lists are an important use-case.

One other possibility is to use a variant of what GHC does for literal strings. Currently
turns into	
	unpackCString "foo"#
where "foo"# is a statically allocate C string, and the "unpackCString" unpacks it lazily.

Maybe we could make a literal [a,b.c] turn into
	unpack [a,b,c]#
is a statically-allocated vector?  See http://hackage.haskell.org/trac/ghc/ticket/5218, which is stalled awaiting brain cycles from someone.

I'm maxed out at the moment.  I'd be very happy if you guys were able to make progress; I'm happy to advise.  Open a ticket, start a wiki page, etc!


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