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50 lines
1.8 KiB
Markdown
50 lines
1.8 KiB
Markdown
6 years ago
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### Action Execution Environment
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Actions and predicates have these variables and functions available to them.
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* All variables and functions defined in the initializer at the beginning of the grammar are available.
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* Labels from preceding expressions are available as local variables, which will have the match result of the labelled expressions.
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A label is only available after its labelled expression is matched:
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```pegjs
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rule = A:('a' B:'b' { /* B is available, A is not */ } )
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```
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A label in a sub-expression is only valid within the sub-expression:
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```pegjs
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rule = A:'a' (B: 'b') (C: 'b' { /* A and C are available, B is not */ })
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```
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* `options` is a variable that contains the parser options.
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* `error(message, where)` will report an error and throw an exception. `where` is optional; the default is the value of `location()`.
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* `expected(message, where)` is similar to `error`, but reports: Expected _message_ but "_other_" found
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* `location()` returns an object like this:
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```js
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{
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start: { offset: 23, line: 5, column: 6 },
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end: { offset: 25, line: 5, column: 8 }
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}
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```
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For actions, `start` refers to the position at the beginning of the preceding expression, and `end` refers to the position after the end of the preceding expression.
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For predicates, `start` and `end` are the same, the location where the predicate is evaluated.
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`offset` is a 0-based character index within the source text.
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`line` and `column` are 1-based indices.
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Note that `line` and `column` are somewhat expensive to compute, so if you need location frequently, you might want to use `offset()` or `range()` instead.
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* `offset()` returns the start offset.
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* `range()` returns an array containing the start and end offsets, such as `[23, 25]`.
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* `text()` returns the source text between `start` and `end` (which will be "" for predicates).
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