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