Need `import`

[JacquesCarette]

PR #117 could be made better, and so would issue #96 if we had import working. There has been much discussion [but apparently never recorded here? Just in the mailing list?] about how to do this.

[zaxtax]

From my own notes from chatting with @ccshan about this suggest whatever solution we have provide

1. Capture-avoiding substitution

I don't know if that is the right term. Basically baz.hk should work.

# foo.hk
normal <~ uniform(0, 1)

# bar.hk
y = normal(0,1)

# baz.hk
import foo
import bar

x <~ y
return (x + normal)

2. Changes to the grammar to distinguish declarations from expressions

[JacquesCarette]

Isn't normal already defined, so that foo.hk should, rather than 'work', return an error? I would consider it a bug, not a feature, if this worked!

If you were to rename normal to something else (like say z), then I would indeed expect baz.hk to work.

However, I do agree that import should be reasonably hygienic. Certainly if foo.hk defined something like y = uniform(0,1), then baz.hk should fail upon import of bar. I'm ok with shadowing for let, but shadowing across files that becomes order-dependent is a bad idea.

[zaxtax]

So something like foo.hk does give me an error, but the following is a perfectly valid program

def sin(x real):
    cos(x)
sin(5)

I think we are in broad agreement, but we should be very clear and precise about what properties we
want and don't want the module system to obey.

[zaxtax]

One property I would like is that we have some way to inline definitions in different files so the simplifier be able to work across those boundaries.

[JacquesCarette]

Why does redefining sin work? Why is that a good thing? Doesn't that just lead to much debugging pain for very little gain?

I purposefully avoiding using the word module. If we want a module system, then we're going to need to spend quite a bit more effort on it - likely more than is currently available.

Nevertheless, I do think we are generally in agreement. Definitely agree that I want to be able to inline definitions from different files.

[zaxtax]

Bumping to get feedback from @ccshan