Engine Configuration Options
============================

{{#include ../links.md}}

A number of other configuration options are available from the [`Engine`] to fine-tune behavior and safeguards.


Compile-Time Language Features
------------------------------

| Method                                                             | Description                                                                            |                 Default                 |
| ------------------------------------------------------------------ | -------------------------------------------------------------------------------------- | :-------------------------------------: |
| `set_optimization_level`<br/>(not available under [`no_optimize`]) | sets the amount of script _optimizations_ performed (see [script optimization])        | [`Simple`][`OptimizationLevel::Simple`] |
| `set_allow_if_expression`                                          | allows/disallows [`if`-expressions]({{rootUrl}}/language/if.md#if-expression)          |                  allow                  |
| `set_allow_switch_expression`                                      | allows/disallows [`switch` expressions]({{rootUrl}}/language/switch-expression.md)     |                  allow                  |
| `set_allow_loop_expressions`                                       | allows/disallows loop expressions                                                      |                  allow                  |
| `set_allow_statement_expression`                                   | allows/disallows [statement expressions]({{rootUrl}}/language/statement-expression.md) |                  allow                  |
| `set_allow_anonymous_fn`<br/>(not available under [`no_function`]) | allows/disallows [anonymous functions]                                                 |                  allow                  |
| `set_allow_looping`                                                | allows/disallows looping (i.e. [`while`], [`loop`], [`do`] and [`for`] statements)     |                  allow                  |
| `set_allow_shadowing`                                              | allows/disallows _[shadowing]_ of [variables]                                          |                  allow                  |
| `set_strict_variables`                                             | enables/disables [_Strict Variables_ mode][strict variables]                           |                disabled                 |
| `set_fast_operators`                                               | enables/disables [_Fast Operators_ mode][fast operators]                               |                 enabled                 |
| `disable_symbol`                                                   | disables a certain [keyword] or [operator] (see [disable keywords and operators])      |                                         |

Beware that these options activate during _compile-time_ only.  If an [`AST`] is compiled on an
[`Engine`] but then evaluated on a different [`Engine`] with different configuration, disallowed
features contained inside the [`AST`] will still run as normal.


Runtime Behavior
----------------

| Method                                                                     | Description                                                                                                                                      |
| -------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------ |
| `set_fail_on_invalid_map_property`<br/>(not available under [`no_object`]) | sets whether to raise errors (instead of returning [`()`]) when invalid properties are accessed on [object maps]                                 |
| `set_default_tag`                                                          | sets the default value of the _custom state_ (which can be obtained via [`NativeCallContext::tag`][`NativeCallContext`]) for each evaluation run |


Safety Limits
-------------

| Method                     |      Not available under       | Description                                                                                                                             |
| -------------------------- | :----------------------------: | --------------------------------------------------------------------------------------------------------------------------------------- |
| `set_max_expr_depths`      |         [`unchecked`]          | sets the maximum nesting levels of an expression/statement (see [maximum statement depth])                                              |
| `set_max_call_levels`      | [`unchecked`], [`no_function`] | sets the maximum number of function call levels (default 50) to avoid infinite recursion (see [maximum call stack depth])               |
| `set_max_operations`       |         [`unchecked`]          | sets the maximum number of _operations_ that a script is allowed to consume (see [maximum number of operations])                        |
| `set_max_variables`        |         [`unchecked`]          | sets the maximum number of [variables] that a script is allowed to define within a single [`Scope`] (see [maximum number of variables]) |
| `set_max_functions`        | [`unchecked`], [`no_function`] | sets the maximum number of [functions] that a script is allowed to define (see [maximum number of functions])                           |
| `set_max_modules`          | [`unchecked`], [`no_modules`]  | sets the maximum number of [modules] that a script is allowed to load (see [maximum number of modules])                                 |
| `set_max_string_size`      |         [`unchecked`]          | sets the maximum length (in UTF-8 bytes) for [strings] (see [maximum length of strings])                                                |
| `set_max_array_size`       |  [`unchecked`], [`no_index`]   | sets the maximum size for [arrays] (see [maximum size of arrays])                                                                       |
| `set_max_map_size`         |  [`unchecked`], [`no_object`]  | sets the maximum number of properties for [object maps] (see [maximum size of object maps])                                             |
| `set_max_strings_interned` |                                | sets the maximum number of [strings] to be interned (if zero, the [strings interner] is disabled)                                       |