Proposal: Add insertAt to Data.Sequence

Andreas Abel abela at
Mon May 30 06:09:49 UTC 2016

On 30.05.2016 00:26, David Feuer wrote:
> No, it would not be feasible to have it return a Maybe; usability
> would go down the drain.

A priori, I cannot see why.  It is easy to make a Maybe-partial function 
"total" by prefixing it with

   fromMaybe (error "the universe is inconsistent")

If you want to insert at the beginning or at the end, I suppose special 
functions are already in place.

Cheers, Andreas

> My preferred total alternative would be to
> drop the bounds check in that spec, so
> insertAt (-1) 99 [1,2,3]  ===  [99,1,2,3]
> insertAt 12 99 [1,2,3] === [1,2,3,99]
> The only potential concern is that trying to insert at an out-of-range
> index seems likely to be a mistake someone might want to get an error
> message about. There are a number of other functions in Data.Sequence
> that are partial in similar ways; I'm very open to making them total,
> but the previous maintainers were concerned about losing useful
> errors.
> On Sun, May 29, 2016 at 6:10 PM, Ivan Lazar Miljenovic
> <ivan.miljenovic at> wrote:
>> On 30 May 2016 at 06:32, David Feuer <david.feuer at> wrote:
>>> I've come up with an implementation of
>>> insertAt :: Int -> a -> Seq a -> Seq a
>>> -- Defined to be equivalent to
>>> insertAt i x xs
>>>    | i < 0 || i > length xs = error "insertAt: index out of range"
>>>    | otherwise = take i xs <> singleton x <> drop i xs
>>> that inserts the given element at the given index with very little
>>> tree restructuring. I came up with the vague notion that it might be
>>> interesting to try to do something like this early last year, but I
>>> wasn't sure how at the time. I largely forgot about it until today,
>>> when someone on StackOverflow pointed to the issue I'd opened and
>>> asked if it would be implemented, presumably because he wants it. What
>>> do y'all think?
>> The partiality is a little troubling; would it be feasible from a
>> usage point of view to have this function return a Maybe?
>> --
>> Ivan Lazar Miljenovic
>> Ivan.Miljenovic at
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Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

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