[Haskell-cafe] exists . a psuedo-standard non-empty list module

[oleg at pobox.com]

Nicolas Frisby wrote:
> Some of the code from the previous wiki link, type-level decimal
> numbers

I would rather advice against type-level decimal numbers, if we are
looking at a lightweight solution. The [complete] code at

        http://www.haskell.org/haskellwiki/Non-empty_list
is Haskell98! It also provides for a head and tail functions -- which
are total and so raise no errors.

Incidentally, inserting NList into the existing Safe.List does not
seem like a good match as NList critically relies on being in a
separate module with a limited export. NList constitutes a security
kernel, whose invariants are to be described explicitly (and
hopefully, automatically proven at some point). For that reason, it is
beneficial to keep module NList separate and compact. It could of
course be placed within Safe.* hierarchy, etc -- as a separate module.


Neil Mitchell wrote:
> Fortunately the problem of pattern match errors is being tackled by at
> least two projects:
>
> * Catch: http://www-users.cs.york.ac.uk/~ndm/projects/catch.php
> * ESC-Haskell: http://www.cl.cam.ac.uk/~nx200/research/escH-hw.ps

I would submit the approach of the lightweight static capabilities (cf
above Wiki link) to be counted as the third project in that area. The
latter has an advantage that it is available in Haskell right now and
requires no extra tools.