[Haskell-cafe] Prolog-style list syntax?
ietf-dane at dukhovni.org
Mon Jun 28 17:02:53 UTC 2021
On Mon, Jun 28, 2021 at 06:17:42PM +1200, Anthony Clayden 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 ) = ...
Yes, there's presently no way to pattern-match a list tail 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?
I'm on the violently against side on the question of `[x : ys]` being
reinterpreted as `(x : ys)`. For any expression `e1, we have a
singleton `[e]`, and there should not be any surprising exceptions to
If something new is to pattern match as `x : ys` inside `` it would
have to be invalid syntax presently. So it could in principle be:
but I'm concerned that this is too likely to be an inadvertent typo,
, y ]
It is probably more useful to catch this sort of error, than to overload
the syntax for head + tail pattern matches.
So on the whole, I'm inclined to accept the status quo. The annoyance,
if any, of having to use `(x : ys)` in pattern matches would not be
solved by adding a third syntax for lists.
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