{ local1 local2 ... -- comment }
or
{ local1 local2 ... }
E.g.,
: max { n1 n2 -- n3 }
n1 n2 > if
n1
else
n2
endif ;
The similarity of locals definitions with stack comments is intended. A
locals definition often replaces the stack comment of a word. The order
of the locals corresponds to the order in a stack comment and everything
after the -- is really a comment.
This similarity has one disadvantage: It is too easy to confuse locals declarations with stack comments, causing bugs and making them hard to find. However, this problem can be avoided by appropriate coding conventions: Do not use both notations in the same program. If you do, they should be distinguished using additional means, e.g. by position.
The name of the local may be preceded by a type specifier, e.g.,
F: for a floating point value:
: CX* { F: Ar F: Ai F: Br F: Bi -- Cr Ci }
\ complex multiplication
Ar Br f* Ai Bi f* f-
Ar Bi f* Ai Br f* f+ ;
Gforth currently supports cells (W:, W^), doubles
(D:, D^), floats (F:, F^) and characters
(C:, C^) in two flavours: a value-flavoured local (defined
with W:, D: etc.) produces its value and can be changed
with TO. A variable-flavoured local (defined with W^ etc.)
produces its address (which becomes invalid when the variable's scope is
left). E.g., the standard word emit can be defined in terms of
type like this:
: emit { C^ char* -- }
char* 1 type ;
A local without type specifier is a W: local. Both flavours of
locals are initialized with values from the data or FP stack.
Gforth supports the square bracket notation of local data structures.
These locals are similar to variable-flavored locals, the size is
specified as a constant expression. A declaration looks
name[ size ]. The Forth expression size is
evaluated during declaration, it must have the stack effect ( --
+n ), giving the size in bytes. The square bracket [ is part
of the defined name.
Local data structures are initialized by copying size bytes from
an address passed on the stack; uninitialized local data structures
(after | in the declaration) are not erased, they just contain
whatever data there was on the locals stack before.
Example:
begin-structure test-struct
field: a1
field: a2
end-structure
: test-local {: foo[ test-struct ] :}
foo[ a1 ! foo[ a2 !
foo[ test-struct dump ;
Gforth allows defining locals everywhere in a colon definition. This poses the following questions: