5.2 Setters and Getters

Given a struct value, the obvious way to access the value of a field is to just refer to it using dot-notation.

For example, for the following struct type:

type ID3V1_Tag =
  struct
  {
    [...]
    char[30] title;
    char[30] artist;
    char[30] album;
    char[4] year;
    [...]
  }

Suppose the find out the year in the tag is wrong, off by two years: the song was release in 1980, not in 1978!. Unfortunately, due to the bizarre way the year is stored in the file (as a sequence of digits encoded in ASCII, non-NULL terminated) we cannot just write:

(poke) tag.year = tag.year + 2
error

Instead, we can use facilities from the standard library and a bit of programming:

(poke) stoca (format ("%d", atoi (catos (tag.year)) + 2), tag.year)

The above line basically transforms the data we want to operate on (the tag year) from the stored representation into a more useful representation (from an array of character digits to an integer) then operates with it (adds two) then converts back to the stored representation. Let’s call this “more useful” representation the “preferred representation”.

A well written pickle should provide getter and setter methods for fields in struct types for which the stored representation is not the preferred representation. By convention, getter and setter methods have the following form:

method get_field preferred_type: { ... }
method set_field (preferred_type val) void: { ... }

Using the get_ and set_ prefixes consistently is very important, because the pokist using your pickle can easily find out the available methods for some given value using tab-completion in the REPL.

For example, let’s add setter and getter methods for the field year in the ID3V1 tag struct above:

type ID3V1_Tag =
  struct
  {
    [...]
    char[4] year;

    method get_year = int: { return atoi (catos (year)); }
    method set_year = (int val) void:
    {
      var str = format "%d", val;
      stoca (str, year);
    }
    [...]
  }

What constitutes the preferred representation of a field is up to the criteria of the pickle writer. For the tag above, I would say the preferred representations for the title, artist, album and year are string for title, artist and album, and an integer for the year.