[Haskell-cafe] syntactic sugar for heterogeneous lists

[Antoine Latter]

On Tue, Apr 12, 2011 at 12:57 PM, Alberto G. Corona <agocorona at gmail.com> wrote:
> Hi Cafe:
> http://hackage.haskell.org/trac/ghc/ticket/1245
>
> I also want some kind of syntactic sugar for H. Lists.
> The absence of syntactic sugar makes heterogeneous list to look scary.
> People in haskell is accostumed to syntactic sugaring, so people think of
> not sugared expressions as second class.
> I´ don't know the details, but it seems that tuples are in the language to
> cover the heterogeneous flexibility thing that homogeneous lists may not
> provide, but they introduce its own inflexibilities; These repeating
> instances for two, three, four and so on tuples makes them artificial and
> repetitive. In its comparison, a complex but unique H. List instance look
> elegant. specially if it is sugarized.
> An alternative to sugarize H.Lists preserving tuples could be to use {} to
> sugarize H. lists as
>    {x,y,z}
> and desugarize it into:
> x :*: y :*: z :*: {}
> Just like [] means empty list, {} would mean HNil, the empty heterogeneous
> list.
>
> But this alternative , if implemented, would soon render tuples  obsolete.
> These (,,) (,,,) constructors may create marginally faster and compact
> structures, but they are much less manageable.
>

You could use quasi-quotes as an way to do this while you explore
building it in to a compiler:

> [hlist|5, "hello", True]

You should be able to steal most of the implementation from
haskell-src-exts-qq. It would work for expressions and patterns. You
could even make a type quasi-quoter if you get ambitious.

Antoine