Re: syntactic sugar for imports and exports

[Xavier Wang <weasley.wx@...>]

I love this idea!

2011/10/20 Tony Finch <dot@dotat.at>:
> Further thoughts on the use+mention problem, where we have to repeat a
> name to use it as a variable and to mention it as a table index or
> function argument. The main examples are:
>
>  local name = name            -- keep name in scope after altering _ENV
>  local name = module.name     -- import unqualified name from module
>  local name = require "name"  -- module linkage
>  { name = name }              -- export variable into table
>
> The usual proposal to improve the first two is as follows, but I'm trying
> to find a more general solution.
>
>  local name in _ENV
>  local name in module
>
> I call the first three "projection" from a table or function into
> variables and the last one "injection" from variables into a table.
>
> It seems to be helpful to think of both projection and injection as
> special kinds of assignment. This emphasizes their duality, and it
> suggests a better syntax for injection.
>
> So the syntax for projection might be like
>
>  foo, bar, zig <- tbl
>
> where "foo, bar, zig" is a list of variable names and "tbl" is an
> expresion. The "<-" is pronounced "from" as in "assigned from" or
> "imported from". This is equivalent to
>
>  foo, bar, zig = tbl.foo, tbl.bar, tbl.zig
>
> except that tbl is evaluated once. A projection statement starting with
> "local" has the obvious translation:
>
>  local foo, bar, zig <- tbl
>  local foo, bar, zig = tbl.foo, tbl.bar, tbl.zig
>
> Injection might be like
>
>  tbl -< foo, bar, zig
>
> which is equivalent to
>
>  tbl.foo, tbl.bar, tbl.zig = foo, bar, zig
>
> with the same conditions as before. Injection needs a different operator
> so you can tell the difference between "x <- y" and "y -< x". The "-<"
> symbol is the syringe operator.
>
> I think on balance it doesn't make sense to support projecting from
> functions directly. This avoids muddling the concepts of indexing and
> calling, but you can still use the __index metamethod if you want. For
> example,
>
>  -- syntactic sugar for require
>
>  library = setmetatable(package.loaded, {
>              __index = function (_, name)
>                          return require(name)
>                        end
>            })
>
>  -- get math and io modules from the library
>
>  local math, io <- library
>
> Tony.
> --
> f.anthony.n.finch  <dot@dotat.at>  http://dotat.at/
> Dover, Wight, Portland, Plymouth: Northwest 4 or 5, increasing 6 at times,
> backing west 3 later. Slight or moderate, occasionally rough in Plymouth at
> first. Showers. Good.
>
>