[plt-scheme] Modules, structs and environments

From: Joel J. Adamson (jadamson at partners.org)
Date: Mon Nov 19 11:15:08 EST 2007

The following message is a courtesy copy of an article
that has been posted to comp.lang.scheme as well.

Howdy Schemers,

There's something I'm not getting about require'ing modules,
environments and how to export struct definitions.  I'm using PLT Scheme
(MzScheme only) 371.  When I import a struct and then start a new REPL,
I get undefined identifier from within the REPL for make-`struct'.

Here's some test code:

;; struct.ss
;; this is a library that creates the struct and exports
;; it.  
(module struct mzscheme
  (provide (struct test-struct (f1 f2)))
  (define-struct test-struct (f1 f2)))

;; run.ss
;; This is "main"
;; I compile this with `mzc -v --exe run run.ss' and then invoke it from
;; the command-line
(module run mzscheme
  (require "struct.ss")

  (define test (make-test-struct

  (display test)


This prints "<struct:test-struct>" (i.e., it works) and then enters the
REPL.  If I `make-test-struct' in the REPL I get "undefined identifier:

The obvious explanation is that the REPL initializes the namespace and
therefore the struct is not imported into the new namespace.  In that
case, should I pass the environment to the REPL, and how would I do
that?  I started defining my own `eval', which seems like either playing
with fire or a sign of a novice ;)

Also I read in a draft of R6RS that run-time identifiers must be
imported with some qualification or flag --- do I need to do that in
mzscheme?  If so, how?  All I've found is require-for-syntax and
require-for-template: is there a require-for-run?

The overall rationale: my real program is to take an input file
(currently composed of scheme code) and evaluate it, and print the
results to an output file or STDOUT.  After I configure the input and
output ports from command line options, starting an REPL seemed to be
the easiest way to do this.  If there's a better way, please let me
know.  I will add custom printers and all that, but that is pretty far
down the road.

Joel J. Adamson
Pediatric Psychopharmacology Research Unit
Massachusetts General Hospital
Boston, MA  02114
(617) 643-1432
(303) 880-3109

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