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_archive/rhai_engine/rhaibook/engine/optimize/side-effects.md

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+Side-Effect Considerations for Full Optimization Level
+======================================================
+
+{{#include ../../links.md}}
+
+All of Rhai's built-in functions (and operators which are implemented as functions) are _pure_
+(i.e. they do not mutate state nor cause any side-effects, with the exception of `print` and `debug`
+which are handled specially) so using [`OptimizationLevel::Full`] is usually quite safe _unless_
+custom types and functions are registered.
+
+If custom functions are registered, they _may_ be called (or maybe not, if the calls happen to lie
+within a pruned code block).
+
+If custom functions are registered to overload [built-in operators], they will also be called when
+the operators are used (in an [`if`] statement, for example), potentially causing side-effects.
+
+```admonish tip.small "Rule of thumb"
+
+* _Always_ register custom types and functions _after_ compiling scripts if [`OptimizationLevel::Full`] is used.
+
+* _DO NOT_ depend on knowledge that the functions have no side-effects, because those functions can change later on and,
+  when that happens, existing scripts may break in subtle ways.
+```