85 lines
2.6 KiB
Markdown
85 lines
2.6 KiB
Markdown
Use Rhai as a Domain-Specific Language (DSL)
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===========================================
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{{#include ../links.md}}
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Rhai can be successfully used as a domain-specific language (DSL).
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Expressions Only
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----------------
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In many DSL scenarios, only evaluation of expressions is needed.
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The `Engine::eval_expression_XXX`[`eval_expression`] API can be used to restrict
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a script to expressions only.
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Disable Keywords and/or Operators
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--------------------------------
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In some DSL scenarios, it is necessary to further restrict the language to exclude certain
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language features that are not necessary or dangerous to the application.
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For example, a DSL may disable the `while` loop altogether while keeping all other statement
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types intact.
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It is possible, in Rhai, to surgically [disable keywords and operators].
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Custom Operators
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----------------
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On the other hand, some DSL scenarios require special operators that make sense only for
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that specific environment. In such cases, it is possible to define [custom operators] in Rhai.
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For example:
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```rust
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let animal = "rabbit";
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let food = "carrot";
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animal eats food // custom operator - 'eats'
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eats(animal, food) // <- the above really de-sugars to this
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```
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Although a [custom operator] always de-sugars to a simple function call,
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nevertheless it makes the DSL syntax much simpler and expressive.
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Custom Syntax
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-------------
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For advanced DSL scenarios, it is possible to define entire expression [_syntax_][custom syntax] -
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essentially custom statement types.
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The [`internals`] feature is needed to be able to define [custom syntax] in Rhai.
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For example, the following is a SQL like syntax for some obscure DSL operation:
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```rust
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let table = [..., ..., ..., ...];
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// Syntax = "calculate" $ident$ $ident$ "from" $expr$ "->" $ident$ ":" $expr$
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let total = calculate sum price from table -> row : row.weight > 50;
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// Note: There is nothing special about the use of symbols; to make it look exactly like SQL:
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// Syntax = "SELECT" $ident$ "(" $ident$ ")" "FROM" $expr$ "AS" $ident$ "WHERE" $expr$
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let total = SELECT sum(price) FROM table AS row WHERE row.weight > 50;
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```
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After registering this custom syntax with Rhai, it can be used anywhere inside a script as
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a normal expression.
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For its evaluation, the callback function will receive the following list of parameters:
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`exprs[0] = "sum"` - math operator
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`exprs[1] = "price"` - field name
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`exprs[2] = Expr(table)` - data source
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`exprs[3] = "row"` - loop variable name
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`exprs[4] = Expr(row.wright > 50)` - expression
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The other identified, such as `"select"`, `"from"`, as as as symbols `->` and `:` are
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parsed in the order defined within the custom syntax.
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