Re: [Haskell-beginners] How best to do this?

Ah, yes, a warning for literals like [5..2] sounds like an excellent idea. Why don't you file a feature request at http://hackage.haskell.org/trac/ghc/ ? -Brent On Wed, May 01, 2013 at 12:35:36PM +0200, Michael Peternell wrote:

@Brent: By reading your mails I got an idea...

how about generating a compiler warning when someone types a literal like [5..2].

"Foo.hs:32: Warning: Literal list [5..2] evaluates to [] because 5 > 2 and the default step size is +1. Replace the literal with the empty list or with [5,4..2] to suppress this warning."

That would be a really beginner-friendly warning.

But I also think that we shouldn't catch this kind of mistake at runtime, because it may be the expected behavior in many cases.

Michael

Am 29.04.2013 um 23:54 schrieb Brent Yorgey:

...

Oh, yes, I suppose it could.

In any case, I am still in favor of the existing semantics -- it is simple and consistent (and sometimes even useful). "Do what I mean"-style semantics with special cases end up generating more pain than they solve.

-Brent

On Mon, Apr 29, 2013 at 02:29:43PM -0400, David McBride wrote:

...

Couldn't it just use fromEnum and compare the integers you get and figure out which is bigger?

On Mon, Apr 29, 2013 at 2:17 PM, Brent Yorgey wrote:

...

But then \x y -> [x .. y] would have to have the type

(Ord a, Enum a) => [a]

whereas now it just has the Enum constraint. Either that or else the notation would work differently for literals vs. expressions but that would be just awful.

-Brent

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