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

[Evan Laforge]

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from haskell-src-exts but it has bugs, it can be out of date, and it's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.

>> 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").