Impure function
An impure function is a function whose behavior depends on external state or whose evaluation produces observable effects beyond its returned value.
Write Powerful Rust Macros
An impure function is a function whose behavior or result can depend on or produce effects beyond its input parameters.
- How it is used
- Distinguishes deterministic computation from functions that interact with databases, systems, logging, or error-producing control flow.
- How deep it goes
- Explains external dependence, nondeterminism, transitive impurity, side effects, logging, and testing implications without formal implementation detail.
- Examples
- Uses deterministic summation, database-dependent summation, logging, macro tests, and panics versus Result as teaching examples.
- What you can do after
- Evaluate function boundaries, isolate transformations in pure functions, and test impure edges with focused integration coverage.
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Terraform in Depth: Infrastructure as Code with Terraform and OpenTofu
An impure function is a Terraform function whose result is not guaranteed to be known during planning or to remain fixed across evaluations.
- How it is used
- Explains why unstable expression results can continually update Terraform resources and prevent early planning of collection sizes.
- How deep it goes
- Defines purity, explains deployment consequences, and stops short of implementing function internals.
- Examples
- Uses uuid and timestamp expressions, resource tags, random resources, and early-planning collection arguments.
- What you can do after
- Diagnose repeated Terraform changes and replace unstable expressions with persisted random-provider resources when appropriate.
Related concepts
- pure functioncontrasts with · A pure function is a function whose output is determined solely by its explicit inputs and whose evaluation has no externally observable side effects.
- actionequivalent to · An action is a choice or operation selected or performed by an actor that can affect a system or environment or produce an outcome.
- pseudo-random number generatorincludes · A pseudorandom number generator is a deterministic computational process that uses an initial seed or internal state to produce a sequence with random-like statistical properties.
- functionkind of · A function is a defined operation that maps inputs to outputs and, when implemented as a callable program unit, may also produce effects.
- terraform resourceused by · A Terraform resource is a declarative configuration construct that describes an infrastructure object and its settings for Terraform to manage.
- static methodcontrasts with · A static method is a function associated with a type or enclosing scope rather than an object instance, and callable without an instance receiver.
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