Node:Eval escapes, Previous:Input File Grammar, Up:Input File Grammar

Eval escapes

The text in eval-escape is Forth code that is evaluated when Vmgen reads the line. You will normally use this feature to define stacks and types.

If you do not know (and do not want to learn) Forth, you can build the text according to the following grammar; these rules are normally all Forth you need for using Vmgen:

text: stack-decl|type-prefix-decl|stack-prefix-decl|set-flag

stack-decl: 'stack ' ident ident ident
type-prefix-decl:
    's" ' string '" ' ('single'|'double') ident 'type-prefix' ident
stack-prefix-decl:  ident 'stack-prefix' string
set-flag: ('store-optimization'|'include-skipped-insts') ('on'|'off')

Note that the syntax of this code is not checked thoroughly (there are many other Forth program fragments that could be written in an eval-escape).

A stack prefix can contain letters, digits, or :, and may start with an #; e.g., in Gforth the return stack has the stack prefix R:. This restriction is not checked during the stack prefix definition, but it is enforced by the parsing rules for stack items later.

If you know Forth, the stack effects of the non-standard words involved are:

stack                 ( "name" "pointer" "type" -- )
                      ( name execution: -- stack )
type-prefix           ( addr u item-size stack "prefix" -- )
single                ( -- item-size )
double                ( -- item-size )
stack-prefix          ( stack "prefix" -- )
store-optimization    ( -- addr )
include-skipped-insts ( -- addr )

An item-size takes three cells on the stack.