"case" of an empty type should have no branches

[Roman Cheplyaka]

* John Meacham <john at repetae.net> [2011-10-10 19:56:59-0700]
> What are you trying to acomplish? A case doesn't necessarily force
> evaluation in haskell depending on the binding pattern. for instance
> 
> case x of _ -> undefined will parse, but the function is still lazy in
> x. it is exactly equivalant to
> 
> quodlibet x = undefined
> 
> If you want to actually enforce that quodlibet _|_ evaluates to _|_
> then you want quodlibet x = x `seq` undefined. Though, that too is
> technically equivalent in a theoretical sense, but may have practical
> benefits when it comes to error messages depending on what you are
> trying to acomplish.

In GHC, you need pseq for that. To quote the docs [1],

  Note that we use pseq rather than seq. The two are almost equivalent,
  but differ in their runtime behaviour in a subtle way: seq can
  evaluate its arguments in either order, but pseq is required to
  evaluate its first argument before its second, which makes it more
  suitable for controlling the evaluation order in conjunction with par. 

[1]:  http://www.haskell.org/ghc/docs/latest/html/users_guide/lang-parallel.html

So, if, for some reason, the second argument of seq will be evaluated
first, you'll get an "undefined" error message.

> On Sun, Oct 9, 2011 at 4:26 AM, Roman Beslik <beroal at ukr.net> wrote:
> > Hi.
> >
> > Why the following code does not work?
> >> data Empty
> >> quodlibet :: Empty -> a
> >> quodlibet x = case x of
> > "parse error (possibly incorrect indentation)"
> >
> > This works in Coq, for instance. Demand for empty types is not big, but they
> > are useful for generating finite types:
> >> Empty ≅ {}
> >> Maybe Empty ≅ {0}
> >> Maybe (Maybe Empty) ≅ {0, 1}
> >> Maybe (Maybe (Maybe Empty)) ≅ {0, 1, 2}
> > etc. Number of 'Maybe's = number of elements. I can replace @Maybe Empty@
> > with @()@, but this adds some complexity.
> >
> > _______________________________________________
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> > Glasgow-haskell-users at haskell.org
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> >
> 
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-- 
Roman I. Cheplyaka :: http://ro-che.info/