[erlang-questions] idea: ml9 (sort of) in the erlang shell

Roberto Saccon <>
Tue Nov 13 17:34:26 CET 2007


Ulf, that is great stuff ! Most of all I liked the idea of plugging in
an X-to-Erlang parser. I have a real use case for that (I am writing a
Javascript to Erlang Parser)

regards

On Nov 13, 2007 1:52 PM, Ulf Wiger <> wrote:
> No one commented on my output filters for the shell.
> Oh well, perhaps it wasn't weird enough... (:
> Personally, I love them. Working with dicts in the shell has
> become much more pleasant.
>
> Here's a more outlandish idea:
>
> Allow for arbitrary syntax to be inserted in the shell, by
> catching lines starting with '@' (only because this is what
> Joe used in his ML9).
>
> Here's an example:
>
> Eshell V5.5.4  (abort with ^G)
> 1> @ syntax(erlang),import(X,Y).
> -module(a).
> -define(X,x).
>
> f() -> ?X.
>
> @
> Env = [{call,1,{atom,1,syntax},[{atom,1,erlang}]},
>        {call,1,{atom,1,import},[{var,1,'X'},{var,1,'Y'}]}]
> String = "-module(a).\n-define(X,x).\n\nf() -> ?X.\n\n"
>
> (This was as far as I have come. The shell only prints out
> what it collected.)
>
> If a '@' is entered (with an optional sequence of erlang
> expressions describing what will follow, terminated by
> a period), the shell will just collect whatever text follows,
> and return this, instead of calling erl_parse:parse_erl_exprs().
>
> My initial thought was that the default behaviour would be
> to preprocess String, accepting macro definitions etc, and
> then expanding macros in the source. This would be configurable
> in the environment:
>
> 2> @ pp(false).
> L = [a,b,c].
> lists:reverse(L).
> @
> [c,b,a]
> 3>
>
> Eventually, this might evolve into accepting a module declaration,
> but that would require first deciding what to do with the module,
> once parsed. Compile to binary and then load it?
>
> A few ideas about what could be done with this:
>
> - by default, no variables would be imported, but selective
>   import might be possible:
>
> 4> L = [1,2,3].
> 5> @ import(L).
> lists:reverse(L)
> @
> [3,2,1]
>
> - It would be easy to also declare which variables to remember,
>   export(X,Y,Z).
>
> - An obvious use of this would be to make it easier to deal with multi-
>   line expressions in the shell. Ever had to copy a multi-line expression
>   *from* the shell, and having to edit away all the garbage?
>
> - QuickCheck uses macros a lot. This could offer a way to use
>   QuickCheck with the useful macros from within the shell.
>   (Mats Cronqvist already wrote a preprocessor that pretty much
>   works for this purpose.)
>
> - There are a few X-to-Erlang parsers (scheme, haskell, ...) that could
>   be plugged in and used with this. Plug in your own parser.
>   (Should it be possible to import/export variables...? Why not?)
>
> I'd have to write some more code to make it reasonably useful,
> but not too much. It won't break any of the existing stuff. (:
>
> Thoughts, flames?
>
> BR,
> Ulf W
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> 
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>



-- 
Roberto Saccon
http://rsaccon.com



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