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Pack onepointfour_basics -- prolog/doc/README_stringy_length.md



  • ../stringy_length.pl (MIT license)
  • [stringy_length.plt](../stringy_length.plt) (0BSD license) Determine length of an atom or an SWI-Prolog string (a "stringy thing"). This is a mashup of Prolog's ISO-standard atom_length/2 and the additional SWI-Prolog string_length/2. Because having several X_length/2 to handle "stringy things" just doesn't make much sense in an anytyped language. Note that the implementation of SWI-Prolog's atom_length/2 and string_length/2 accepts atoms and strings equally.

Loading the module and running its tests (in SWI-Prolog)

Please refer to the README.md file, but in short:

?- assertz(file_search_path(library,'/foo/bar/prolog_code/unpacked')).
?- use_module(library('onepointfour_basics/stringy_length.pl')).
?- load_test_files([]).
?- run_tests.


  • `stringy_length(+Stringy,?Length)`
  • `stringy_length(+Stringy,?Length,@Tuned)`
  • `stringy_length_type(+Stringy,?Length,?Type)`
  • `stringy_length_type(+Stringy,?Length,?Type,@Tuned)` All predicates throw if:
  • Stringy is unbound;
  • Stringy is bound but not a stringy (i.e. not an atom or string);
  • Length is not an integer if it is bound. The additional input-only-argument Tuned can be set to either hard or soft to control behaviour for negative Length and bad Type.

    If Tuned = hard the predicates throw instead of failing if:

    • In case Length is bound and < 0.
    • In case Type is bound and not one of atom or string.


Determining length of a string:

?- stringy_length("Hello",L).
L = 5.

Determining length of an atom:

?- stringy_length('Hello',5).

Verifying length of a string:

?- stringy_length("Hello",100).

What does it say anout negative length?

?- stringy_length("Hello",-1).

The above is a reasonably "logical" response, but you may want more discipline:

?- stringy_length("Hello",-1,hard).
ERROR: check failed : 'domain' error (the culprit is outside the required domain)
ERROR:    message   : the value should fulfill "integer that is >= 0"-ness
ERROR:    culprit   : -1

Additionally finding the type:

?- stringy_length_type(hello,Length,Type).
Length = 5,
Type = atom.

or accepting the type:

?- stringy_length_type(hello,Length,atom).
Length = 5.

or not as the case may be:

?- stringy_length_type(hello,Length,string).

Using hard mode is always recommended:

?- stringy_length_type(hello,Length,gark,hard).
ERROR: check failed : 'domain' error (the culprit is outside the required domain)
ERROR:    message   : the value should fulfill "stringy_typeid"-ness
ERROR:    culprit   : gark