[Haskell-cafe] Syntax extension - adding import support to let/where bindings

[Oliver Charles]

On Wed, Aug 5, 2015 at 6:08 PM Evan Laforge <qdunkan at gmail.com> wrote:

> On Wed, Aug 5, 2015 at 9:55 AM, Oliver Charles <ollie at ocharles.org.uk>
> wrote:
> > It makes me sad if we can't progress the language on the grounds that
> > people's attempts at parsing the source code themselves would break. If
> you
> > want to know all the imports, then we should be providing this
> information
> > through tools for people to consume.
>
> It's not whether or not there's a tool, there already is.  It's that
> the tool must be more complicated.  For example, we can get imports
> from haskell-src-exts but it has bugs, it can be out of date, and it's
> slower.  Or ghc -M... which doesn't have those problems.  So maybe
> it's not really a serious objection.
>

This is a problem that shouldn't even exist. If you want a tool that
supports GHC's extensions then we should be *using GHC* - it is a library
after all. haskell-src has a need to exist as a non-GHC-dependent parser,
and that's great - but for parsing GHC specific code we should be able to
just use GHC. The fact that that isn't the case right now is a failing on
our parts, but one that we seem to be actively fixing (c.f.
ghc-exactprint).

>> On the other hand, lots of languages have a "local open" feature like
> >> this.  I think many of them make you first import the module, and then
> >> you can "open" it in a local scope.  This would address both my "parse
> >> the whole file for imports" objection and the "what about instances",
> >> because module importing would be unchanged.
> >
> > Indeed, this could be a path forward. I'm not really familiar with any
> > languages that do this, could you link to some examples of how this
> works in
> > other languages?
>
> I was thinking of agda.. though it's only from memory so I could be
> wrong.  Or perhaps it was cayenne... one of those dependently typed
> languages models modules as records, and then has syntax to dequalify
> record access.  Rust has a full-on nested module system, but I seem to
> recall you have to declare a link dependency on an external crate
> ("import"), and then separately import the symbols from it ("use").
>

Indeed, Agda does have stuff like that - though it does require more
syntax. It is a pretty sophisticated import system, though .

*ocharles*
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