Elmord's Magic Valley

Software, lingüística e rock'n'roll. Às vezes em Português, sometimes in English.

Modules in Hel (and Chez Scheme)

2019-04-18 22:41 -0300. Tags: comp, prog, pldesign, lisp, scheme, hel, fenius, in-english

Today I implemented a simple mechanism for importing modules in Hel. Basically, you can write:

import foo/bar/baz

and it will look for a file named foo/bar/baz.hel, load it as a module containing the bindings defined in the file, and expose the module as baz to the calling code. So if foo/bar/baz.hel defines a function hello, then after you import foo/bar/baz, you can call the function as baz.hello(). Alternatively, you can import the module with a different name using:

import foo/bar/baz as whatever

and access the bindings like whatever.hello().

That's all there is to it, which means there's a lot of things missing from the system. But I decided it was best to do the simple thing1 and leave the more complex details of the module system for a later phase.

Oh, one more trick: if the module file is not found, Hel tries to import a Scheme module with the given name. So you can actually say import chezscheme and get all the bindings from the host Scheme. (Except some of the names are inacessible, since there is currently no way to use ? or - in Hel identifiers. We'll see how to fix that in the future.

Modularizing the interpreter

Incidentally, I also started an attempt to split the interpreter (a single 1420-line Scheme file) into modules. I still haven't merged those changes into the master repository, and I'm still not sure if it's a good idea. In principle it'd be great for organization. The problem is that the RnRS/Chez module system is annoying to use.

For instance, you have to list all bindings you want to export in the library declaration. This is especially annoying for records. For example, if you declare a record:

(define-record-type Foo (fields x y))

this will generate a record descriptor Foo, a constructor make-Foo, a type predicate Foo?, and two accessors Foo-x and Foo-y. That's all nice and fun, but you have to export each of the generated identifiers individually if you want to use them in other modules.

Another annoyance is that Chez does not seem to provide a mechanism to run the REPL from the environment of the module. You can switch the interaction environment to the exported bindings of a module, but there does not seem to be a way to switch to the environment within the module, to call non-exported functions, etc. The workaround I found was to split all modules into a library definition file (say, utils.sls), containing just:

(library (utils)
  (export binding1 binding2 binding3 ...)
  (import (chezscheme))
  (include "utils.scm"))

and the module code proper, in a separate file utils.scm. In this way, I can load the module code directly in Geiser or in the REPL, outside the module system. Note also that the the library definition file only imports (chezscheme) (so we can use the include form); all other imports are directly in the .scm file, so the .scm file will load properly by itself.

Even so, it is annoying to reload libraries, because you have to reload the users of each library manually too.

A smaller annoyance is that the code takes longer to compile when split into libraries. This is not a problem if you compile before execution, but makes running the code without a separate compilation step (chezscheme --script hel.scm) slower to start.

Yet another annoyance is that in the R6RS library syntax, all definitions must precede all expressions, so you have to move initialization code to the end of the module. [Addendum: this (mis)feature does not seem to be shared by R7RS library syntax. As much as I've learned to appreciate many good aspects of R6RS, it seems to me that library syntax is just better in R7RS.]

In the end, a middle-ground solution may be to avoid R6RS libraries entirely, and just create a single main file which includes the others, all in the same namespace. It's not as elegant, but it makes development easier. [Addendum: one benefit of the .sls/.scm split is that it's easy to switch to the non-library organization by just including all .scm files directly and ignoring the .sls files.]

Todos and remarks

When you write import foo/bar, where to look for the foo/bar.hel file? Currently the interpreter looks in the current directory, but that's far from ideal. A better option would be to search relative to the file where the import occurs. That would be an improvement, but still somewhat annoying: if I have a project with files foo.hel, dir1/bar.hel and dir2/baz.hel, I want to be able to load either of these files individually in the REPL and for each module to be able to import any other module in the project using the same name. What I really want is a notion of a project root to search from. One possibility would be to have a __project__.hel file (or something similar) at the project root. When looking for imports, the implementation tries to find the __project__.hel file up the directory hierarchy. If it's found, the directory where the file is is the project root. If not, the project root is the directory where the importing file is. This is vaguely similar to Python's __main__.py, except there would be only one project file per project (not per directory, which would destroy the idea of a single project root).

There is still no syntax to import individual bindings from a module, i.e., the equivalent of Python's from mod import foo, bar. Maybe we can just use Python's syntax (except (foo, bar) would have to be in parentheses, due to restrictions of Hel's syntax).

There is also no syntax to import all bindings from a module, i.e., Python's from foo import *; we can't use * because that's an infix operator, and the syntax doesn't and won't special-case individual commands (remember that one of the goals of the syntax is not to have hardcoded keywords). Maybe from foo import all(), and also things like from foo import except(foo, bar). I don't know.

Why foo/bar instead of foo.bar? Because I thought that import foo.bar might give the impression that the bindings are to be accessed as foo.bar.hello() (like Python) instead of just bar.hello() (as Hel does). And why I wanted this semantics? Because I was unsure what foo would be when you import foo.bar: a module containing just bar? What if I import foo later? What if foo contains a binding bar itself? Does the module foo have to exist for me to be able to import foo.bar? To avoid all these questions ("do the simple thing"), I decided it would be simpler to import the module without having to deal with the whole hierarchy, and make it available as just bar; and I thought the syntax with / suggested that better.

_____

1 "When in doubt, do the simple thing" has been a sort of mantra in this project. This has helped me avoid analysis paralysis and keep making progress, even though I know eventually I will have to go back and change/improve things. (Note though that the mantra is "do the simple thing", not "do the simplest thing".)

Comentários / Comments (0)

Deixe um comentário / Leave a comment

Main menu

Posts recentes

Comentários recentes

Tags

em-portugues (213) comp (131) prog (64) life (46) in-english (41) unix (33) pldesign (32) lang (30) random (28) about (26) mind (24) mundane (22) lisp (22) fenius (17) web (17) ramble (16) img (13) rant (12) hel (12) privacy (10) scheme (9) freedom (8) esperanto (7) copyright (7) academia (7) bash (7) lash (7) music (7) home (6) mestrado (6) shell (6) conlang (5) misc (5) etymology (4) latex (4) worldly (4) emacs (4) book (4) php (4) politics (4) editor (4) kbd (3) tour-de-scheme (3) wrong (3) c (3) network (3) film (3) android (3) security (3) poem (2) cook (2) llvm (2) treta (2) comic (2) lows (2) philosophy (2) physics (2) perl (1) en-esperanto (1) audio (1) wm (1) german (1) translation (1) kindle (1) pointless (1)

Elsewhere

Quod vide


Copyright © 2010-2019 Vítor De Araújo
O conteúdo deste blog, a menos que de outra forma especificado, pode ser utilizado segundo os termos da licença Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International.

Powered by Blognir.