Lyndsy Simon

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OpenSCAD Import System: A Proposal

Having recently acquired a 3D printer, I've been diving in to 3D modeling. While Fusion 360 seems to be the leader in that area generally, I've fallen for OpenSCAD. I appreciate its use of code as the primary (read "only") design environment because it mirrors the way I tend to think about problems. This has the added benefits of making parametrized models being a core feature instead of an afterthought, and readable diffs for files under version control with git.

Over the past few days I've been working hard at reaching competency with OpenSCAD and have come to the point where I feel like I'm able to offer an informed perspective on some of its strengths and limitations. The biggest "rough edge" I've run into thus far is dealing with imports.

Note that this is very much a first draft, and that at this point is really more of a "suggestion" than a "proposal". I'm mostly interested in getting the idea down in writing so it can be discussed if anyone else is interested.

Current State & Limitations

OpenSCAD has two provisions for dependency management: include and use. include includes the entirety of the referenced file, while use brings in only modules and functions defined in the global context of the target file.

use is the workhorse here, as it allows you to keep your context as clean as possible while pulling in only the things you need from elsewhere. In contrast, where include really shines is in dependency injection: because the contents of the referenced file is bodily included, it can be used to define a set of variables to be referened in the including context.

The main limitation of use is that it brings all modules and functions from the included context into the including context. For complex libraries, this could mean quite a large number of objects! This seems to be handled by others by breaking down libraries into a number of files, each with a narrow purpose.

Proposal for Improvement

In short, I'd like more granular control over what gets imported. I propose two new builtins, from and import, inspired largely by Python.

import

use should be deprecated in favor of a new builtin, import, with the following behavior:

The .scad extension should be inferred for the argument if necessary. For the statement import <foo>, resolution should occur in the following order:

  1. the file foo in the folder in which the design was opened
  2. the file foo.scad in the folder in which the design was opened
  3. the file foo in the OpenSCAD library folder
  4. the file foo.scad in the OpenSCAD library folder

A means of explicitly including or excluding modules and functions from import should be implemented. This could be as simple as ignoring those beginning with and underscore (i.e., importing only those functions and modules matching the regex ^[^_].*$). Alternatively, special variables could be used to define inclusion/exclusion:

To include only foo and bar:

$export = ["foo", "bar"];

module foo() { ... }
module bar() { ... }
module baz() { ... }

Functionally equivalent would be to exclude baz:

$private = ["baz"];

module foo() { ... }
module bar() { ... }
module baz() { ... }

from

from effectively be the extended form of import, allowing the inclusion of only explicitly specified modules and funtions from the included context.

Given my_library.scad:

module foo() { ... }
module bar() { ... }
module baz() { ... }

to include only foo:

from <my_library> import "foo";

to include both foo and bar:

from <my_library> import ["foo", "bar"];

Notably, using from should ignore the usage of the special variables $export and $private described in the previous section. Modules and functions that are either not included or explicitly excluded from the import builtin should be accessible via from.

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