Tachikoma
f7e8d4b82b
Adds generator support per the plan in issue 57: - environment.h: isGenerator/generatorFn on a GEN call's own environment, isEachLoop and forGeneratorEnv on a FOR EACH/DO EACH loop's own environment. - runtime.c: splits akbasic_runtime_prev_environment() into akbasic_runtime_detach_environment() (return to parent without releasing) and akbasic_runtime_release_environment() (give variables and the pool slot back, on any environment); prev_environment() is now the two in sequence. akbasic_runtime_call_function() refuses to call a GEN like an ordinary function. - verbs.c/verbs.h/scanner: new keywords GEN, EMIT, EACH, IN and the compound verb "END GEN" (built by a new akbasic_parse_end(), the same trick akbasic_parse_print() uses for PRINT #). - parser_commands.c: akbasic_parse_gen() (modelled on multi-line DEF), akbasic_parse_end(), and EACH branches in akbasic_parse_for()/ akbasic_parse_do(). - runtime_generator.c (new): akbasic_cmd_gen, akbasic_cmd_emit, akbasic_cmd_end_gen, and the invoke/pump/release machinery FOR EACH, DO EACH, NEXT and LOOP share. EMIT walks up to the nearest isGenerator ancestor rather than assuming it is standing directly in the GEN's own call frame, because a GEN body may nest its own FOR/DO/GOSUB around an EMIT -- the issue's own ROOMOBJECTS example does exactly that. - runtime_commands.c/runtime_structure.c: EACH branches in cmd_for/cmd_do, matching EACH branches in cmd_next/cmd_loop, and forGeneratorEnv release on every path that can abandon a live generator (EXIT, a NEXT that pops for a mismatched loop variable). Deviates from the plan in one place: FunctionDef gained an isGenerator flag (not in the plan's field list) because refusing a GEN called like a function has to happen before anything is pushed. Relying on EMIT's own isGenerator check for that case doesn't work: akbasic_runtime_call_function() drives its own step loop the same way akbasic_runtime_pump_generator() does, and a BASIC-level error inside that loop is swallowed by process_line_run() as reported-but-not-propagated, so the call would silently "succeed" with a meaningless return value instead of failing. Also: a zero-argument parameter list is not supported by the DEF/GEN parameter parser this reuses (a pre-existing limitation, not generator-specific); every generator in the tests takes at least one parameter as a result. Tests: tests/generators.c (pool exhaustion under repeated EXIT, calling a GEN like a function, EMIT outside a GEN, self-recursion, sibling/nested invocations) and tests/language/flowcontrol/generators_*.bas -- the issue's own ROOMOBJECTS example in both loop shapes, an empty generator, non-numeric EMIT, nested/interleaved invocations, and three error-path golden cases. Docs: control-flow chapter 4 gets a GEN/EMIT/FOR EACH/DO EACH section, the verb reference gets GEN/EMIT/END GEN entries and updated FOR/DO/NEXT/LOOP/EXIT rows, and architecture chapter 14 documents the detach/release split and the two-environment generator invocation. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
7.6 KiB
4. Control flow
IF ... THEN ... ELSE
10 A# = 5
20 IF A# = 5 THEN PRINT "FIVE" ELSE PRINT "NOT FIVE"
FIVE
The condition is a whole expression, so AND, OR and NOT all work in one:
10 A# = 5
20 IF A# > 0 AND A# < 10 THEN PRINT "IN RANGE"
IN RANGE
Everything after THEN on the line belongs to the condition, which matters as soon
as you put several statements on a line:
| Line | Condition | What runs |
|---|---|---|
IF C THEN A : B |
true | A, B |
IF C THEN A : B |
false | nothing |
IF C THEN A ELSE B : D |
true | A |
IF C THEN A ELSE B : D |
false | B, D |
The remainder always belongs to whichever arm was written last.
Blocks
BEGIN and BEND make an IF span lines:
10 A# = 5
20 IF A# = 5 THEN BEGIN
30 PRINT "IN THE BLOCK"
40 PRINT "STILL IN IT"
50 BEND
60 PRINT "AFTER"
IN THE BLOCK
STILL IN IT
AFTER
When the condition is false every line up to the BEND is skipped.
FOR ... NEXT
10 FOR I# = 1 TO 5
20 PRINT I#
30 NEXT I#
1
2
3
4
5
STEP sets the stride, and a negative one counts down:
10 FOR I# = 10 TO 0 STEP -2
20 PRINT I#
30 NEXT I#
10
8
6
4
2
0
The counter is an ordinary variable and the body may assign to it. Two things to know:
- The counter does not survive the loop. It lives in the loop's own scope, so reading it afterwards gives zero. On a C128 it keeps its final value.
- A step that overshoots runs the body one extra time.
FOR I = 1 TO 9 STEP 3runs with 1, 4, 7 and 10. Both are recorded as known defects; see Chapter 13.
EXIT leaves the loop early:
10 FOR I# = 1 TO 100
20 IF I# = 5 THEN EXIT
30 NEXT I#
40 PRINT "OUT"
OUT
DO ... LOOP
The condition can go on either end, or neither:
10 I# = 0
20 DO WHILE I# < 3
30 PRINT I#
40 I# = I# + 1
50 LOOP
0
1
2
10 I# = 0
20 DO
30 PRINT I#
40 I# = I# + 1
50 LOOP UNTIL I# = 3
0
1
2
A condition on the DO is tested before the body, so the body may run zero times. A
condition on the LOOP is tested after, so it runs at least once. DO with no
condition at all loops forever until an EXIT or a GOTO leaves it.
EXIT works here too.
GEN ... EMIT and FOR EACH / DO EACH
A GEN is a subroutine that hands back more than one value, one at a time, instead of
returning once. It looks like a multi-line DEF, except its body runs EMIT where a
function would RETURN, and it ends in END GEN rather than RETURN:
10 GEN COUNTUP(N#)
20 FOR I# = 1 TO N#
30 EMIT I#
40 NEXT I#
50 END GEN
60 FOR EACH V# IN COUNTUP(3)
70 PRINT V#
80 NEXT V#
1
2
3
FOR EACH is what runs a GEN: it calls COUNTUP(3) the way FOR EACH V# IN says,
and each EMIT inside the generator's body becomes one trip through the loop, with V#
holding whatever was emitted. When the generator's body reaches END GEN with nothing
left to emit, the loop ends -- there is no separate "no more values" check to write.
DO EACH ... LOOP does the same thing:
10 GEN COUNTUP(N#)
20 FOR I# = 1 TO N#
30 EMIT I#
40 NEXT I#
50 END GEN
60 DO EACH V# IN COUNTUP(3)
70 PRINT V#
80 LOOP
1
2
3
A GEN's body is ordinary BASIC: it may hold its own FOR, DO, IF or GOSUB
around the EMITs, and even invoke another GEN with its own FOR EACH/DO EACH --
the same generator invoked with different arguments, nested or side by side, is not
recursion. A GEN invoking itself from within its own currently-running body is
refused, the same way it would be an error to call a function that has not returned yet
by name from inside its own body without meaning to recurse.
EXIT leaves a FOR EACH/DO EACH loop early, exactly as it does a plain FOR or
DO, and the generator it was consuming stops there -- nothing forces the rest of it to
run just because the loop started it:
10 GEN COUNTUP(N#)
20 FOR I# = 1 TO N#
30 EMIT I#
40 NEXT I#
50 END GEN
60 FOR EACH V# IN COUNTUP(100)
70 PRINT V#
80 IF V# = 3 THEN EXIT
90 NEXT V#
100 PRINT "STOPPED"
1
2
3
STOPPED
A GEN shares its namespace with DEF -- the same name cannot be both -- but it is not
a function and cannot be called like one: X# = COUNTUP(3) is refused, because nothing
about an ordinary call means "resume where the last EMIT left off." FOR EACH/DO EACH is the only thing that consumes a GEN.
GOTO and GOSUB
10 GOSUB 100
20 PRINT "BACK"
30 END
100 PRINT "IN THE SUBROUTINE"
110 RETURN
IN THE SUBROUTINE
BACK
RETURN goes back to the line after the GOSUB.
Labels
A label is a name with no type suffix. It marks a line, and anything that takes a line number takes a label instead:
10 GOTO SETUP
20 PRINT "SKIPPED"
100 LABEL SETUP
110 PRINT "ARRIVED"
ARRIVED
Labels are filed before the program runs, so a forward GOTO works. This is worth
using for its own sake: a program written with labels is immune to RENUMBER, because
there is no number to rewrite.
Follow that one step further and the numbers stop earning their keep at all. A program in a file can leave them out — see Chapter 2 — and then every branch it makes is by name:
GOSUB GREET
GOTO FINISH
PRINT "NOT REACHED"
LABEL GREET
PRINT "HELLO"
RETURN
LABEL FINISH
PRINT "DONE"
HELLO
DONE
Nothing in that program can be broken by inserting a line into it, which is the whole argument.
ON
ON picks the nth target from a list, counting from one:
10 CHOICE# = 2
20 ON CHOICE# GOTO 100, 200, 300
30 PRINT "CHOICE WAS OUT OF RANGE"
40 END
100 PRINT "FIRST"
110 END
200 PRINT "SECOND"
210 END
300 PRINT "THIRD"
SECOND
Out of range is not an error — it falls through to the next statement, which is what
lets you write the check as the line after. ON ... GOSUB works the same way and
returns.
Trapping errors
TRAP sends an error to a handler instead of stopping the program:
10 TRAP HANDLER
20 DIM Q#(2)
30 PRINT Q#(9)
40 PRINT "CARRIED ON"
50 END
100 LABEL HANDLER
110 PRINT "CAUGHT " + ERR(ER#) + " ON LINE " + EL#
120 RESUME NEXT
CAUGHT Out Of Bounds ON LINE 30
CARRIED ON
Two variables are set when the trap fires: ER# is the error code and EL# is
the line it happened on. ERR(ER#) gives the message text. On a C128 these are called
ER and EL with no suffix; this dialect has no bare variable names.
Chapter 15 is the complete list of what ER# can hold and
what each code means. The short version is that the interpreter's own codes are 512 to
519 and are the only numbers worth comparing against; two codes render as the same
ERR() text, so compare the number rather than the text.
RESUME comes in three forms:
| Form | Where it goes |
|---|---|
RESUME |
back to the line that failed, and tries again |
RESUME NEXT |
on to the line after the one that failed |
RESUME (line) |
to a line you name |
Bare RESUME only terminates if the handler fixed whatever was wrong — that is what
it is for.
TRAP with no argument turns trapping off. An error inside a handler is reported
normally rather than re-entering it, so a broken handler cannot loop forever.
STOP, END and CONT
STOP stops the program and returns to the prompt; CONT resumes from there. END
stops it without arming CONT. QUIT ends the interpreter itself.