Feat: UI / UX

docs/control_flow.md

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+# Control Flow
+
+A drum pattern that plays the same sound on every step is not very interesting. You want kicks on the downbeats, snares on the backbeats, hats filling the gaps. Control flow is how you make decisions inside a step.
+
+## if / else / then
+
+The simplest branch. Push a condition, then `if`:
+
+```forth
+step 4 mod 0 = if kick s . then
+```
+
+Every fourth step gets a kick. The rest do nothing. Add `else` for a two-way split:
+
+```forth
+step 2 mod 0 = if
+  kick s 0.8 gain .
+else
+  hat s 0.3 gain .
+then
+```
+
+These are compiler syntax -- you won't find them in the dictionary. Think nothing of it.
+
+## ? and !?
+
+When you already have a quotation, `?` executes it if the condition is truthy:
+
+```forth
+{ snare s . } coin ?
+```
+
+`!?` is the opposite -- executes when falsy:
+
+```forth
+{ hat s 0.2 gain . } coin !?
+```
+
+These pair well with `chance`, `prob`, and the other probability words:
+
+```forth
+{ rim s . } fill ?              ;; rim only during fills
+{ 0.5 verb } 0.3 chance ?      ;; occasional reverb wash
+```
+
+## ifelse
+
+Two quotations, one condition. The true branch comes first:
+
+```forth
+{ kick s . } { hat s . } step 2 mod 0 = ifelse
+```
+
+Reads naturally: "kick or hat, depending on whether it's an even step."
+
+```forth
+{ c3 note } { c4 note } coin ifelse
+saw s 0.6 gain .                ;; bass or lead, coin flip
+```
+
+## pick
+
+Choose the nth option from a list of quotations:
+
+```forth
+{ kick s . } { snare s . } { hat s . } step 3 mod pick
+```
+
+Step 0 plays kick, step 1 plays snare, step 2 plays hat. The index is 0-based.
+
+```forth
+{ c4 } { e4 } { g4 } { b4 } step 4 mod pick
+note sine s 0.5 decay .
+```
+
+Four notes cycling through a major seventh chord, one per step.
+
+## case / of / endof / endcase
+
+For matching a value against several options. Cleaner than a chain of `if`s when you have more than two branches:
+
+```forth
+step 8 mod case
+  0 of kick s . endof
+  4 of snare s . endof
+endcase
+```
+
+Steps 0 and 4 get sounds. Everything else falls through to `endcase` and nothing happens.
+
+A fuller pattern:
+
+```forth
+step 8 mod case
+  0 of kick s 0.9 gain .    endof
+  2 of hat s 0.3 gain .     endof
+  4 of snare s 0.7 gain .   endof
+  6 of hat s 0.3 gain .     endof
+  hat s 0.15 gain .
+endcase
+```
+
+The last line before `endcase` is the default -- it runs when no `of` matched. Here it gives unmatched steps a ghost hat.
+
+The `of` value can be any expression:
+
+```forth
+step 16 mod case
+  0         of kick s .               endof
+  3 1 +     of snare s .              endof
+  2 4 *     of kick s . snare s .     endof
+endcase
+```
+
+## times
+
+Repeat a quotation n times. `@i` holds the current iteration (starting from 0):
+
+```forth
+4 { @i 4 / at hat s . } times   ;; four hats, evenly spaced
+```
+
+Build chords:
+
+```forth
+3 { c4 @i 4 * + note } times
+sine s 0.4 gain 0.5 verb .      ;; c4, e4, g#4
+```
+
+Subdivide and accent:
+
+```forth
+8 {
+  @i 8 / at
+  @i 4 mod 0 = if 0.7 else 0.2 then gain
+  hat s .
+} times
+```
+
+Eight hats per step. Every fourth one louder.
+
+## Putting It Together
+
+A basic drum pattern using `case`:
+
+```forth
+step 8 mod case
+  0 of kick s .                     endof
+  2 of { hat s . } often            endof
+  4 of snare s .                    endof
+  6 of { rim s . } sometimes        endof
+  { hat s 0.15 gain . } coin ?
+endcase
+```
+
+Kicks and snares on the strong beats. Hats and rims show up probabilistically. The default sprinkles ghost hats.
+
+A melodic step that picks a scale degree and adds micro-timing:
+
+```forth
+{ c4 } { d4 } { e4 } { g4 } { a4 } step 5 mod pick
+note
+
+step 3 mod 0 = if
+  0 0.33 0.66 at          ;; triplet feel on every third step
+then
+
+saw s 0.4 gain 0.3 decay 0.2 verb .
+```
+
+A `times` loop paired with `case` for a drum machine in one step:
+
+```forth
+4 {
+  @i case
+    0 of kick s .                  endof
+    1 of hat s 0.3 gain .          endof
+    2 of snare s .                 endof
+    3 of { rim s . } 0.5 chance    endof
+  endcase
+  @i 4 / at
+} times
+```
+
+Four voices, four sub-positions, one step.