Write down that the left operand decides integer or float arithmetic

No behaviour change, by decision. `A# * 0.45` is 0 and `0.45 * A#` is 1.35,
because every operator branches on `self->valuetype` and converts the right
operand to match. It is inherited from the Go reference, a C128 promotes to
float instead, and this interpreter is at least consistent about it -- so a
dialect saying the left operand wins is a defensible position, and changing it
to promotion would alter the result of every mixed expression in every existing
program.

**The defect was that nobody said so.** Chapter 3's "Numbers" did not mention
it, Chapter 13 did not list it among the differences, and nothing fails when a
program gets it wrong -- it computes something else and carries on. The game in
examples/ lost its per-level speed increase to `5.6 + LEVEL# * 0.45` evaluating
to a flat 5.6, and bled velocity out of every bounce through `0 - BLVX%(B#)`
quantising to whole pixels. Both read correctly. Neither produced a diagnostic.

Now said in three places: a section in Chapter 3 with the demonstration and the
two rules that keep a program out of it (put the float on the left, put the
answer somewhere with a `%` on it), a row in Chapter 13 naming it as the
difference from 7.0 most likely to turn a working listing into a quietly wrong
one, and the reasoning on value.h where the operators are declared.

tests/value_arithmetic.c pins it in both directions across multiply and
subtract, with a comment saying it is the documented contract rather than an
accident -- so promotion becomes a decision somebody takes deliberately rather
than a change that could slip in under a passing suite.

TODO.md section 9 item 4, struck as a documentation outcome.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

TODO.md

@@ -2667,7 +2667,19 @@ reduced against `build/basic`, the stdio build, unless it says otherwise.
 
    The tee half is three lines and is worth doing whatever is decided about the rest.
 
-4. **Mixed-type arithmetic is decided by the left operand alone, and nothing says so.**
+4. ~~**Mixed-type arithmetic is decided by the left operand alone, and nothing says so.**~~
+   **Done, as documentation, by decision.** The behaviour is unchanged: a dialect is
+   allowed to say the left operand wins and this one is consistent about it, and promotion
+   would change the result of every mixed expression in every existing program. What was
+   missing was anybody saying so. Now: a section in `docs/03-the-language.md` with the
+   two-line demonstration and the two rules that keep a program out of it, a row in
+   `docs/13-differences.md` naming it as the difference from 7.0 most likely to turn a
+   working listing into a quietly wrong one, and the reasoning on `include/akbasic/value.h`
+   where the operators are declared.
+
+   `tests/value_arithmetic.c` pins it in both directions, so **promotion is now a decision
+   somebody has to take deliberately** rather than a change that could slip in. The
+   original report:
 
    ```basic
    A# = 3
@@ -2700,7 +2712,18 @@ reduced against `build/basic`, the stdio build, unless it says otherwise.
    The cheap fix is documentation: a paragraph in chapter 3 and a row in chapter 13. The other
    fix is promotion, which would be a real change of semantics and wants its own decision.
 
-5. **A drawing that spans a 256-line batch boundary is torn, not merely transient.** §5 and
+5. ~~**A drawing that spans a 256-line batch boundary is torn, not merely transient.**~~
+   **Done, as documentation.** No fix is possible without changing what a host owns -- the
+   budget is the host's and the present is the host's -- so what was needed was for the
+   consequence to be written down beside the advice it qualifies. `docs/13-differences.md`
+   now says that "redraw it every frame" is not sufficient on its own and carries the
+   measured numbers, `docs/06-graphics.md` says it where `SSHAPE` is introduced, and
+   `docs/14-architecture.md` explains the mechanism from the step loop's side and points at
+   the `TI#` synchronisation a program has to build.
+
+   **Untested, and deliberately:** it needs the real frontend, a clock and a timing
+   window, and a test that reproduced it would be reproducing a race. Said here rather than
+   left looking like an oversight. The original report: §5 and
    chapter 13 record that drawing does not persist across frames. What neither says is that a
    single uninterrupted sequence of drawing verbs longer than one `akbasic_runtime_run()` budget
    comes back **half drawn**, because the present in the middle of it discards the buffer — so

docs/03-the-language.md

@@ -49,6 +49,36 @@ Literals may be written in hexadecimal with a `0x` prefix. A leading zero is *no
 octal — `010` is ten, because a leading zero in a listing is far more often padding
 than a base.
 
+### The left operand decides whether the arithmetic is integer or float
+
+**Not the wider of the two, and not the destination.** An integer on the left of an
+operator converts the right-hand side to an integer, which throws away its fraction:
+
+```basic
+A# = 3
+PRINT A# * 0.45
+PRINT 0.45 * A#
+```
+
+```output
+0
+1.350000
+```
+
+That is the same expression written the other way round, and the answers differ by
+everything. A C128 promotes to float instead; this interpreter does not, and it is
+consistent about it — see [Chapter 13](13-differences.md).
+
+**Nothing fails when you get it wrong.** The program computes something else and carries
+on, which is what makes it worth learning early rather than meeting later. Two rules keep
+you out of it:
+
+- **Put the float on the left.** `0.45 * LEVEL#`, not `LEVEL# * 0.45`. Likewise
+  `0.0 - V%` rather than `0 - V%` to negate a float, which is otherwise quantised to a
+  whole number.
+- **Put the answer somewhere that can hold it.** A float expression assigned to a `#`
+  variable truncates: `T# = 4.2 / 5.6` is `0`.
+
 ## Strings
 
 Strings are up to 255 characters and are written in double quotes. There is no

docs/13-differences.md

@@ -42,6 +42,18 @@ There is no escape character.
 Integers are 64-bit and floats are IEEE doubles, so `PRINT 1.5` gives `1.500000`. A
 leading zero is not octal; `0x` is hexadecimal.
 
+### Mixed arithmetic follows the left operand, not the wider type
+
+**A C128 promotes to float. This does not.** An integer on the left converts the right
+operand to an integer and throws its fraction away, so `A# * 0.45` is `0` where
+`0.45 * A#` is `1.35`. It is consistent, it is inherited from the Go interpreter this was
+ported from, and **nothing fails when you get it wrong** — the program computes something
+else and carries on.
+
+[Chapter 3](03-the-language.md#the-left-operand-decides-whether-the-arithmetic-is-integer-or-float)
+has the two rules that keep you out of it. This is the difference from 7.0 most likely to
+turn a working listing into a quietly wrong one.
+
 ### Structures are an addition
 
 BASIC 7.0 has no records at all. `TYPE`/`END TYPE`, `DIM X@ AS T`, the `@` suffix, `.` and
@@ -133,6 +145,16 @@ interpreter's error code, which bears no relation to a Commodore error number. P
 - **Drawing does not persist across frames.** The verbs draw straight to the renderer,
   so anything drawn is overwritten on the next frame unless the program redraws it.
   This is a defect, not a design.
+- **And a redraw has to *fit*.** The host runs a fixed number of source lines and then
+  presents the frame, and presenting discards the drawing buffer — so a sequence of
+  drawing verbs longer than one batch comes back **half drawn**, with an `SSHAPE` at the
+  end capturing only what was issued since the present. "Redraw it every frame" is
+  therefore not sufficient advice on its own. Measured against the standalone frontend's
+  256 lines a batch: after synchronising to a jiffy edge, 220 lines of drawing survive a
+  capture and 250 do not. The only way a program can see the boundary is to watch `TI#`,
+  which is refreshed once per batch — so the step on which it changes is the first step
+  of one. [Chapter 18](18-tutorial-breakout-artwork.md#step-4-find-the-frame-boundary)
+  builds a pacing routine out of exactly that.
 - **`CHAR` ignores its colour argument** and needs a text device with a cursor.
 
 ## Sound

include/akbasic/value.h

@@ -6,6 +6,16 @@
  * exactly, including the parts that look wrong -- see TODO.md section 12 for the
  * catalogue and the reason they are not fixed yet.
  *
+ * **The left operand decides whether an operation is done in integers or in
+ * floats**, and this is deliberate rather than an oversight: every operator
+ * branches on `self->valuetype` and converts the right operand to match, so an
+ * integer on the left truncates a float on the right. `A# * 0.45` is 0 and
+ * `0.45 * A#` is 1.35. It is inherited from the reference
+ * (basicvalue.go:190-193), a C128 promotes to float instead, and nothing fails
+ * when a program gets it wrong -- it computes something else and carries on.
+ * Documented in docs/03-the-language.md and docs/13-differences.md, and pinned
+ * in tests/value_arithmetic.c so that changing it has to be a decision.
+ *
  * Every binary operator takes a `scratch` value the caller has drawn from an
  * environment's pool and an out-parameter `dest`. Most operators set `*dest` to
  * `scratch` and fill it; akbasic_value_math_plus sets `*dest` to `self` when

tests/value_arithmetic.c

@@ -146,6 +146,57 @@ static void test_pool_starts_empty(void)
 			AKBASIC_ERR_BOUNDS);
 }
 
+/**
+ * @brief The left operand alone decides integer or float arithmetic.
+ *
+ * **This is the documented contract, not an accident**, which is the whole
+ * reason it is asserted: every operator branches on `self->valuetype` and
+ * converts the right operand to match, so an integer on the left truncates a
+ * float on the right and `A# * 0.45` is 0 where `0.45 * A#` is 1.35.
+ *
+ * It is inherited from the Go reference (basicvalue.go:190-193) and a C128
+ * promotes to float instead. It may well be the wrong choice -- what makes it
+ * expensive is that **nothing fails**, so a program computes something else and
+ * carries on; a game written against this interpreter lost a per-level speed
+ * increase and bled velocity out of every bounce before either was noticed.
+ *
+ * Changing it to promotion is a real change of dialect semantics and wants its
+ * own decision. This test is here so that it has to be one: docs/03 and docs/13
+ * describe the behaviour below, and a change that makes them wrong fails here
+ * first. TODO.md section 9 item 4.
+ */
+static void test_left_operand_decides_the_type(void)
+{
+    akbasic_Value *out = NULL;
+
+    /* Integer on the left: the float on the right is truncated to 0. */
+    set_int(&A, 3);
+    set_float(&B, 0.45);
+    TEST_REQUIRE_OK(akbasic_value_math_multiply(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->valuetype, AKBASIC_TYPE_INTEGER);
+    TEST_REQUIRE_INT(out->intval, 0);
+
+    /* Float on the left: the integer on the right is widened. */
+    set_float(&A, 0.45);
+    set_int(&B, 3);
+    TEST_REQUIRE_OK(akbasic_value_math_multiply(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->valuetype, AKBASIC_TYPE_FLOAT);
+    TEST_REQUIRE_FEQ(out->floatval, 1.35);
+
+    /* The same asymmetry in subtraction, which is how a program negates. */
+    set_int(&A, 0);
+    set_float(&B, 6.4);
+    TEST_REQUIRE_OK(akbasic_value_math_minus(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->valuetype, AKBASIC_TYPE_INTEGER);
+    TEST_REQUIRE_INT(out->intval, -6);
+
+    set_float(&A, 0.0);
+    set_float(&B, 6.4);
+    TEST_REQUIRE_OK(akbasic_value_math_minus(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->valuetype, AKBASIC_TYPE_FLOAT);
+    TEST_REQUIRE_FEQ(out->floatval, -6.4);
+}
+
 int main(void)
 {
     akbasic_Value *out = NULL;
@@ -310,6 +361,7 @@ int main(void)
 
     test_maximum_length_string();
     test_pool_starts_empty();
+    test_left_operand_decides_the_type();
 
     return akbasic_test_failures;
 }