Customizable debug toString / repr / representation

[GreyCat]

While debugging format's definition by visualizing the object tree, it's quite often helpful to have a concise string representation of the object. It's akin to #299, but that one is generated fully automatically, and this one is something that a format developer decides would be the best to display as a string.

WebIDE since the dawn of times had a WebIDE-specific key -webide-presentation, which could be used on type level to show one-line long summary of the object even without opening its tree entry — but this facility is:

• WebIDE-only
• requires some substantial effort on visualizer side to support (effectively, reimplementing what compiler is already doing)
• makes it impossible to move that to other languages, as these expressions use specifically JavaScript-rendered names (i.e. lowerCamelCase instead of lower_underscore_case).

KSY proposal

Add a type-level key to-string, which will list a KS expression returning string. People are encouraged to add contents of various important attributes to this string.

Example:

seq:
  - id: foo
    type: strz
  - id: bar
    type: strz
to-string: |
  "foo=" + foo + ", bar=" + bar

Of course, it looks super ugly compared to interpolated strings in -webide-representation, so probably a templated/interpolated strings support in KS expression language would be beneficial.

Obviously, this is a separate task, so I will create separate issue for that.

Implementation

If present, string will override "to string" / "repr"-style rendering generated automatically with #299, i.e. for Java example above will yield:

public String toString() {
    return "foo=" + foo + ", bar=" + bar;
}

or Ruby will yield:

def inspect
  "foo=" + foo + ", bar=" + bar
end

[GreyCat]

Given 2 thumbs up, I went ahead an implemented simple proof-of-concept in the compiler (targeting Ruby only so far) + one test (kaitai-io/kaitai_struct_tests@6ed8894) + spec for Ruby.

[KOLANICH]

Add a type-level key to-string, which will list a KS expression returning string. People are encouraged to add contents of various important attributes to this string.

My experience with python __repr__ is a bit painful when dealing with repr strings. Often I have to implement a middle layer, returning tuples of tuples (key, value) and then transform that tuple into a repr str. So it may make sense not to specify the whole repr string format, but to split it into 3 parts: fields to be displayed (by default all the fields), field repr format ({key}={value} by default), and overall format ({name}<{fields}>, `separator=", "), and allow to either setup them as strings (maybe even the ones parsed from the blob itself, but we need to make that secure!) or completely override in runtime.

[demberto]

3 years later, any progress on this?

[KOLANICH]

0 new commits referring to-string in the compiler: https://github.com/kaitai-io/kaitai_struct_compiler/search?q=%22to-string%22&type=commits
and 1 referring it in C++ runtime: kaitai-io/kaitai_struct_cpp_stl_runtime@da0d91f
The impl in the WebIDE is broken.