[Haskell] Re: Views in Haskell

Stefan Monnier monnier at iro.umontreal.ca
Mon Jan 22 18:39:13 EST 2007

> Provoked by a discussion with Don Syme, and after some helpful
> conversations at POPL, I have finally written up a proposal for adding
> "view patterns" to Haskell.  We've wanted views for a long time, but they
> have never made it, into GHC at any rate.  This proposal is a very
> lightweight (and hence, I hope, cost-effective) proposal for
> a view-like mechanism.

>         http://hackage.haskell.org/trac/haskell-prime/wiki/ViewPatterns

It's nicely simple&general, but I find it at the same time disappointing,
because a pattern guard would give you almost the same result without much
syntactic overhead.
I.e. in the above page you give an example with a "regexp" matching
function, which is great when you match only against one regexp, but as soon
as you try to match against several patterns, it seems you're stuck with
either matching each pattern in turn, or resorting to an external lex tool.

The way I see views, they should allow you to implement
lex/yacc/burg directly.


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