[Haskell-cafe] Re: [m..n] question

Jon Fairbairn jon.fairbairn at cl.cam.ac.uk
Mon Sep 22 05:46:40 EDT 2008

"Richard A. O'Keefe" <ok at cs.otago.ac.nz> writes:

> Erlang's equivalent of [m..n] is lists:seq(M, N),
> which is currently defined to raise an exception when N < M.
> In particular, lists:seq(1, N) returns a list of length N
> when N > 0, but not when N = 0.
> I'm currently arguing that lists:seq(1, 0) should be [],
> not an exception.  Oddly enough, I'm being beaten over the
> head with Haskell, of all things.
> In Haskell,
> "The sequence enumFromTo e1 e3 is the list [e1,e1+1,e1+2,...e3].
>  The list is empty if e1 > e3."
> It is being claimed that the reason for this is that "exceptions
> are problematic" in Hasell, so the Haskell designers went out of
> their way to make this function total whether it made sense or not.

I'm pretty sure that's not true.  I'd like to be able to say
"I know, I was there", but although I was there it was so
long ago that my memory isn't clear.  But it's clearly the
case that [5 .. 6] is [5, 6] (length 2) and [5 .. 5] has to
be [5] (length 1), so it's not unreasonable to expect that
[5, 4] be [] (length 0) -- having the induction extend down
to there makes most sense.  I'm fairly certain it was
arguments about induction and what constituted sensible
behaviour rather than our admitted dislike for exceptions
that carried the argument.

Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk

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