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Add readme section about signals

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lash 2023-04-01 20:39:06 +01:00
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@ -10,8 +10,8 @@ Original motivation was to create a simple templating renderer for USSD clients,
The VM defines the following opcode symbols: The VM defines the following opcode symbols:
* `BACK` - Return to the previous execution frame (will fail if at top frame). It leaves to the state of the execution layer to define what "previous" means. * `BACK` - Return to the previous execution frame (will fail if at top frame). It leaves to the state of the execution layer to define what "previous" means.
* `CATCH <symbol> <signal>` - Jump to symbol if signal is set (see `signal` below). * `CATCH <symbol> <signal>` - Jump to symbol if signal is set (see `signals` below).
* `CROAK <signal>` - Clear state and restart execution from top if signal is set (see `signal` below). * `CROAK <signal>` - Clear state and restart execution from top if signal is set (see `signals` below).
* `LOAD <symbol> <size>` - Execute the code symbol `symbol` and cache the data, constrained to the given `size`. Can be exposed with `MAP` within scope, * `LOAD <symbol> <size>` - Execute the code symbol `symbol` and cache the data, constrained to the given `size`. Can be exposed with `MAP` within scope,
* `RELOAD <symbol>` - Execute a code symbol already loaded by `LOAD` and cache the data, constrained to the previously given `size` for the same symbol. * `RELOAD <symbol>` - Execute a code symbol already loaded by `LOAD` and cache the data, constrained to the previously given `size` for the same symbol.
* `MAP <symbol>` - Expose a code symbol previously loaded by `LOAD` to the rendering client. Roughly corresponds to the `global` directive in Python. * `MAP <symbol>` - Expose a code symbol previously loaded by `LOAD` to the rendering client. Roughly corresponds to the `global` directive in Python.
@ -28,6 +28,7 @@ Loaded symbols are not automatically exposed to the rendering client. To expose
The associated content of loaded symbols may be refreshed using the `RELOAD` opcode. `RELOAD` only works within the same constraints as `MAP`. However, updated content must be available even if a `MAP` precedes a `RELOAD` within the same frame. The associated content of loaded symbols may be refreshed using the `RELOAD` opcode. `RELOAD` only works within the same constraints as `MAP`. However, updated content must be available even if a `MAP` precedes a `RELOAD` within the same frame.
### External symbol optimizations ### External symbol optimizations
Only `LOAD` and `RELOAD` should trigger external code side-effects. Only `LOAD` and `RELOAD` should trigger external code side-effects.
@ -40,6 +41,14 @@ In an effort to prevent leaks from unnecessary external code executions, the fol
Any code compiler or checked **should** generate an error on any orphaned `LOAD` or `MAP` symbols as described above. Any code compiler or checked **should** generate an error on any orphaned `LOAD` or `MAP` symbols as described above.
### Signals
Signal may be set when executing of external code symbols, and may be used as a simple exception mechanism.
The signal flag arguments should only set a single flag to be tested. If more than one flag is set, the first flag matched will be used as the trigger.
## Rendering ## Rendering
The fixed-size output is generated using a templating language, and a combination of one or more _max size_ properties, and an optional _sink_ property that will attempt to consume all remaining capacity of the rendered template. The fixed-size output is generated using a templating language, and a combination of one or more _max size_ properties, and an optional _sink_ property that will attempt to consume all remaining capacity of the rendered template.