# C Compatibility with other Prolog dialects

This chapter explains issues for writing portable Prolog programs. It was started after discussion with Vitor Santos Costa, the leading developer of YAP Prolog187http://yap.sourceforge.net/ YAP and SWI-Prolog have expressed the ambition to enhance the portability beyond the trivial Prolog examples, including complex libraries involving foreign code.

Although it is our aim to enhance compatibility, we are still faced with many incompatibilities between the dialects. As a first step both YAP and SWI will provide some instruments that help developing portable code. A first release of these tools appeared in SWI-Prolog 5.6.43. Some of the facilities are implemented in the base system, others in the library `library(dialect.pl)`.

- The Prolog flag dialect is an unambiguous and fast way to find out which Prolog dialect executes your program. It has the value `swi` for SWI-Prolog and `yap` on YAP.

- The Prolog flag version_data is bound to a term `swi(Major, Minor, Patch, Extra)`

- Conditional compilation using `:- if(Condition)` ...`:- endif` is supported. See section 4.3.1.2.

- The predicate expects_dialect/1 allows for specifying for which Prolog system the code was written.

- The predicates exists_source/1 and source_exports/2 can be used to query the library content. The require/1 directive can be used to get access to predicates without knowing their location.

- The module predicates use_module/1, use_module/2 have been extended with a notion for `import-except' and `import-as'. This is particularly useful together with reexport/1 and reexport/2 to compose modules from other modules and mapping names.

- Foreign code can expect `__SWI_PROLOG__` when compiled for SWI-Prolog and `__YAP_PROLOG__` when compiled on YAP.

:- expects_dialect(+Dialect)
  This directive states that the code following the directive is written for the given Prolog Dialect. See also dialect. The declaration holds until the end of the file in which it appears. The current dialect is available using prolog_load_context/2.

  The exact behaviour of this predicate is still subject to discussion. Of course, if Dialect matches the running dialect the directive has no effect. Otherwise we check for the existence of `library(dialect/Dialect)` and load it if the file is found. Currently, this file has this functionality:

  - Define system predicates of the requested dialect we do not have.

  - Apply goal_expansion/2 rules that map conflicting predicates to versions emulating the requested dialect. These expansion rules reside in the dialect compatibility module, but are applied if prolog_load_context(dialect, Dialect) is active.

  - Modify the search path for library directories, putting libraries compatible with the target dialect before the native libraries.

  - Setup support for the default filename extension of the dialect.
exists_source(+Spec)
  Is true if Spec exists as a Prolog source. Spec uses the same conventions as load_files/2. Fails without error if Spec cannot be found.
source_exports(+Spec, +Export)
  Is true if source Spec exports Export, a predicate indicator. Fails without error otherwise.

## Section Index