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Enable custom syntax without internals.

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This commit is contained in:
Stephen Chung
2020-07-22 17:05:13 +08:00
parent 35374f5b3b
commit abf66850f6
8 changed files with 98 additions and 131 deletions
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31
doc/src/engine/dsl.md
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@@ -11,7 +11,7 @@ Expressions Only
In many DSL scenarios, only evaluation of expressions is needed.
The `Engine::eval_expression_XXX`[`eval_expression`] API can be used to restrict
The [`Engine::eval_expression_XXX`][`eval_expression`] API can be used to restrict
a script to expressions only.
@@ -21,8 +21,7 @@ Disable Keywords and/or Operators
In some DSL scenarios, it is necessary to further restrict the language to exclude certain
language features that are not necessary or dangerous to the application.
For example, a DSL may disable the `while` loop altogether while keeping all other statement
types intact.
For example, a DSL may disable the `while` loop while keeping all other statement types intact.
It is possible, in Rhai, to surgically [disable keywords and operators].
@@ -54,31 +53,29 @@ Custom Syntax
For advanced DSL scenarios, it is possible to define entire expression [_syntax_][custom syntax] -
essentially custom statement types.
The [`internals`] feature is needed to be able to define [custom syntax] in Rhai.
For example, the following is a SQL like syntax for some obscure DSL operation:
For example, the following is a SQL-like syntax for some obscure DSL operation:
```rust
let table = [..., ..., ..., ...];
// Syntax = "calculate" $ident$ $ident$ "from" $expr$ "->" $ident$ ":" $expr$
// Syntax = calculate $ident$ $ident$ from $expr$ -> $ident$ : $expr$
let total = calculate sum price from table -> row : row.weight > 50;
// Note: There is nothing special about the use of symbols; to make it look exactly like SQL:
// Syntax = "SELECT" $ident$ "(" $ident$ ")" "FROM" $expr$ "AS" $ident$ "WHERE" $expr$
// Note: There is nothing special about those symbols; to make it look exactly like SQL:
// Syntax = SELECT $ident$ ( $ident$ ) FROM $expr$ AS $ident$ WHERE $expr$
let total = SELECT sum(price) FROM table AS row WHERE row.weight > 50;
```
After registering this custom syntax with Rhai, it can be used anywhere inside a script as
a normal expression.
For its evaluation, the callback function will receive the following list of parameters:
For its evaluation, the callback function will receive the following list of inputs:
`exprs[0] = "sum"` - math operator
`exprs[1] = "price"` - field name
`exprs[2] = Expression(table)` - data source
`exprs[3] = "row"` - loop variable name
`exprs[4] = Expression(row.wright > 50)` - expression
* `inputs[0] = "sum"` - math operator
* `inputs[1] = "price"` - field name
* `inputs[2] = Expression(table)` - data source
* `inputs[3] = "row"` - loop variable name
* `inputs[4] = Expression(row.wright > 50)` - filter predicate
The other identified, such as `"select"`, `"from"`, as as as symbols `->` and `:` are
parsed in the order defined within the custom syntax.
Other identifiers, such as `"calculate"`, `"from"`, as well as symbols such as `->` and `:`,
are parsed in the order defined within the custom syntax.