6.11 Interpretation and Compilation Semantics

The interpretation semantics of a (named) word are what the text interpreter does when it encounters the word in interpret state. It also appears in some other contexts, e.g., the execution token returned by ' word identifies the interpretation semantics of word (in other words, ' word execute is equivalent to interpret-state text interpretation of word).

The compilation semantics of a (named) word are what the text interpreter does when it encounters the word in compile state. It also appears in other contexts, e.g, POSTPONE word compiles18 the compilation semantics of word.

Most words have default compilation semantics: compile the execution semantics (stack effect ( -- )). But a number of words have other compilation semantics, documented for the individual word (including its stack effect).

The standard also talks about execution semantics. In the standard it never differs from the interpretation semantics if both are defined, but one or both of them may not be defined. Gforth makes no difference between interpretation and execution semantics, so these terms are used interchangeably.

In Gforth (since 1.0) all words have defined interpretation/execution semantics. For many words that have no defined interpretation nor execution semantics in the standard (e.g., if), the interpretation/execution semantics in Gforth are to perform the compilation semantics.

In the standard, execution semantics are used to define interpretation and compilation semantics by default: By default, the interpretation semantics of a word are to execute its execution semantics, and the compilation semantics of a word are to compile, its execution semantics.19

Unnamed words (see Anonymous Definitions) cannot be encountered by the text interpreter, ticked, or postponed. Such a word is represented by its xt (see Tokens for Words), and the behaviour when this xt is executed is called its execution semantics.

You can change the semantics of the most-recently defined word:

immediate ( ) core “immediate”

Make the compilation semantics of a word be to execute the execution semantics.

compile-only ( ) gforth-0.2 “compile-only”

Mark the last definition as compile-only; as a result, the text interpreter and ' will warn when they encounter such a word.

restrict ( ) gforth-0.2 “restrict”

A synonym for compile-only

By convention, words with non-default compilation semantics (e.g., immediate words) often have names surrounded with brackets (e.g., ['], see Execution token).

Note that ticking (') a compile-only word gives a warning (“<word> is compile-only”).



In standard terminology, “appends to the current definition”.


In standard terminology: The default interpretation semantics are its execution semantics; the default compilation semantics are to append its execution semantics to the execution semantics of the current definition.