The syntax [a, b..c] in general produces a list which starts with “a", followed by “b", going

up until reaching (possibly including) c in step sizes of (b - a).

(For simplicity’s sake, I only described non-decreasing lists)

So it is logical that a step size of 0 produces an infinite list, when [1,1..1] is given.

Notice that [1,1..1] is not the same as [1..], but "repeat 1”.

Csongor

On 17 Mar 2016, at 02:58, Krisztian Pinter <pin.terminator@gmail.com> wrote:

Hello,

I noticed some odd behaviour with list comprehensions.

[1..1] == [1]

BUT

[1,1..1] == [1..]

I noticed this while writing a Clean program, but it seems Haskell inherited this as well.

In the case of integer lists with step size >= 0 the up_list function[1] is used:

up_list :: Integer -> Integer -> Integer -> [Integer]

up_list x0 delta lim = go (x0 :: Integer)

                    where

                        go x | x > lim   = []

                             | otherwise = x : go (x+delta)

In the case of [1,1..1] x0 == lim, so go will recurse infinitely, producing an infinite list.

I think the reasonable behaviour would be [1,1..1] == [1]. Is there a reason it doesn't work like this?

[1] http://hackage.haskell.org/package/base-4.8.2.0/docs/src/GHC.Enum.html#up_list

Thanks,

Krisztián

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