Data structures ¶

I pushed to the public repositories the mbfl changes that implement data structures; it is nothing flashy; I have enough features that I can start to use it and see how it goes.

I already know that I will probably change the identifiers in the api, but for now that is the way it is: I started with the idea of implementing a bit of metaobject protocol, with data structures representing slots (their type, how to access values) but then I did nothing of it; but it looked fun to call the core object class mbfl_standard_object and its metaclass mbfl_standard_class. Ha! Ha!

I will probably change them to mbfl_default_object and mbfl_default_class and leave mop ambitions to an external library.

The way it is now, it feels similar to the r6rs records syntactic layer, but simpler; we can define a class and we get: a constructor function, a predicate function, accessor and mutator functions for each field. There is no composition of object constructors: if we want a constructor that calls its superclass constructor and so on, we have to implement it ourselves.

Here is how it looks:

mbfl_standard_class_declare(color)
mbfl_standard_object_declare(dark_slate_blue)

mbfl_standard_class_define _(color) _(mbfl_standard_object) 'color' red green blue
color_define _(dark_slate_blue) 72 61 139

declare RED
color_red_var RED _(dark_slate_blue)
"$RED" ⇒ 72

Notice the fancy underscore syntax:

_(dark_slate_blue) → mbfl_datavar(dark_slate_blue)

it makes the code so much more readable! I love it! Use with care!

All the accessors store the field value in a given Bash variable; there is no dot–syntax or similar.

Both the class constructor mbfl_standard_class_define and the instance constructors like color_define accept positional parameters; the “rest” parameters are, respectively, field names and field values. I’m not happy with it: in my experience with Scheme languages, rest parameters make it harder to maintain an api². I could use an index array:

mbfl_declare_index_array_varref(color_field_names, (red green blue))
mbfl_standard_class_define _(color) _(mbfl_standard_object) 'color' _(color_field_names)

but there would be more to type; I could define an m4 macro to make it a bit more descriptive and take away some characters:

mbfl_standard_field_specs_declare(color_field_names, red, green, blue)
mbfl_standard_class_define _(color) _(mbfl_standard_object) 'color' _(color_field_names)

this looks similar to what one would code in the C language: if we want to pass a list to a function, we have to define an array. I do not like having more to type, but I also want better maintainability. I will think of it.

Single inheritance is supported:

mbfl_standard_class_declare(transparent_color)
mbfl_standard_object_declare(trans_dark_slate_blue)

mbfl_standard_class_define _(transparent_color) _(color) 'transparent_color' transparency
transparent_color_define   _(trans_dark_slate_blue) 72 61 139 50

declare RED TRANSPARENCY
transparent_color_red_var          RED          _(trans_dark_slate_blue)
transparent_color_transparency_var TRANSPARENCY _(trans_dark_slate_blue)

"$RED"          ⇒ 72
"$TRANSPARENCY" ⇒ 50

Right now there are no class attributes, but I want them. For example: some classes are not meant to be instantiated; some classes are not meant to be subclassed.

gnu Emacs font locking ¶

For quite some time now I’ve had an Emacs file which adds to shell-script mode some font locking of mbfl functions and macros; I cannot live without it anymore; I have not distributed it. It all amounts to prepare a call to font-lock-add-keywords at the top–level.

I have been using gnu Emacs for 25+ years, but I am still a beginner when it comes to Emacs Lisp coding; I am sure the code is not of great quality.

I am at a point in which including this code in mbfl’s package makes sense. I will do it.

And stuff ¶

The code grows complex and I feel the need to have some sort of error descriptor; it is impossible to have exceptions in Bash; in some situations we can just print an error message to stderr and hope to catch it when running a test suite.

I don’t know what to do.