eyeprolog

ISO atomic conversion

How do atoms, characters, codes and numbers relate?

Standard Prolog has a small family of conversion predicates: join and split atoms, turn an atom into characters or character codes, read a number from characters, and find fragments inside an atom. Most of them work in more than one direction. This example exercises each one and proves every result.

Run it in the playground · Source

The question

Given the atom eyeprolog, what can the ISO conversion predicates tell us? Which prefix joins with prolog to make it, what are the characters of eye, which code is the letter λ, and which three-character pieces does eyeprolog contain?

The rules

report(joined, Atom) :-
  atom_concat(eye, prolog, Atom).

report(prefix, Prefix) :-
  atom_concat(Prefix, prolog, eyeprolog).

report(decoded_character, Char) :-
  char_code(Char, 955).

report(fragment, fragment(Before, Part, After)) :-
  sub_atom(eyeprolog, Before, 3, After, Part).

atom_concat/3 joins two atoms, or with the whole known, finds the missing part. char_code/2 relates a character to its Unicode code point. sub_atom/5 relates an atom to a fragment, with the number of characters before and after it.

The answer

eyeprolog examples/iso-atomic-conversion.pl

report(joined, eyeprolog).
report(prefix, eye).
report(characters, "eye").
report(codes, [65, 90]).
report(decoded_character, 'λ').
report(number, 42).
report(fragment, fragment(0, eye, 6)).
...
report(fragment, fragment(6, log, 0)).

13 answers in all, including all seven three-character fragments of eyeprolog. The character list [e, y, e] is printed as "eye", the double-quoted notation for the same list.

The reason

eyeprolog --proof examples/iso-atomic-conversion.pl

step(report(prefix, eye), rule(2), ['Prefix' = eye], [atom_concat(eye, prolog, eyeprolog)]).
step(report(decoded_character, 'λ'), rule(5), ['Char' = 'λ'], [char_code('λ', 955)]).
step(char_code('λ', 955), builtin, [], []).

Each report rests on one ISO built-in call, recorded with its result filled in. The prefix eye is justified because atom_concat(eye, prolog, eyeprolog) holds.

The check

condition('C1', resolution, ok, 13).
condition('C5', re_decision, ok, 12).
condition('C7', relevance, ok, 38).
verdict(checked).

The checker replays the 13 rule steps and independently recomputes the 12 built-in conversions. Every result is confirmed; nothing is trusted.

Try it

Add report(float, F) :- number_chars(F, ['3', '.', '5']). and check that it gives 3.5.
Add report(split, Before-After) :- atom_concat(Before, After, ab). It enumerates all three ways to split ab, including the empty atom ''.
Change the fragment length from 3 to 4 and predict the number of fragments.

Learn more in The Art of EyeProlog, Chapter 35.

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