[Haskell-cafe] Prolog-style list syntax?

[Isaac Elliott]

I'd definitely be against what you propose; as you pointed out it
introduces a syntax ambiguity, which I think should be avoided where
possible.

On Mon, 28 Jun 2021, 4:19 pm Anthony Clayden, <anthony.d.clayden at gmail.com>
wrote:

>
> One of the annoying (to me) inconsistencies with Haskell syntax is that
> `[  ]` means 'here comes a list'; but for the most common way to write a
> list, you can't use `[  ]`:
>
> >    elem x []        = False
> >    elem x ( y: ys ) = ...
>
> Prolog syntax has lists, signalled by `[   ]`: [], [1], [2, 3], [4, 5, 6],
> [[7, 8], [9, 10, 11]] are all valid list literals. For cons it also uses
> `[   ]` syntax
>
> >    elem x [y | ys ] = ...
>
> `[ x, y, z | zs ]` is also valid; pattern match or build a list with at
> least 3 elements, where `zs` is the tail -- 4th element onwards (if any).
> That structure is particularly ugly in Haskell syntax.
>
> Neither `|` inside `[   ]` in a pattern nor a comma-list with `|` inside
> `[   ]` is valid H98 syntax, nor valid in GHC extensions AFAICT.
>
> In an expression, `[ x | xs ]` could be taken as a list comprehension,
> which is I guess why Haskell didn't follow Prolog. I've come across another
> theorem-prover that follows Prolog style, except it uses `:` instead of `|`
> inside `[   ]`. Unfortunately ...
>
> >    elem x [ y: ys ] = ...
>
> Is valid syntax, where `:` is cons, so this is a nested list. I'm thinking
> that's relatively uncommon, and means the same as `[ ( y: ys ) ]`.
>
> Would anyone be violently pro or anti changing the meaning of `:`
> appearing at top level in `[   ]` to mean cons-tail?
>
> AntC
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20210628/a27a69fa/attachment.html>