Guard
A guard is a Boolean condition attached to a clause, pattern, or expression that restricts when that construct applies.
Learn Rust in a Month of Lunches
A match guard is an additional Boolean condition attached to a Rust match arm after its pattern matches.
- How it is used
- Refines match arms when a pattern alone does not express the required condition.
- How deep it goes
- Explains syntax, Boolean shorthand, arm ordering, and optional macro expansion without descending into compiler internals.
- Examples
- Uses married-and-children tuples, color tuples, and a matches! macro with a constant Boolean guard.
- What you can do after
- Apply match guards to select arms using additional Boolean conditions and recognize why later arms may not run.
Learn Haskell by Example
A guard is a conditional expression attached to a function or pattern definition that selects a case when its Boolean condition holds.
- How it is used
- Selects function behavior across Boolean cases, especially when nested if-then-else expressions become difficult to read.
- How deep it goes
- Defines syntax, Boolean matching, ordering, and default cases, but does not descend into compiler implementation.
- Examples
- Demonstrates character rotation for lowercase, uppercase, and non-letter inputs.
- What you can do after
- Build it from scratch by replacing nested conditionals with ordered Boolean cases and a default guard.
Tiny Python Projects: Learn coding and testing with puzzles and games
A guard is a conditional clause in a comprehension that determines which iterated elements are included in the resulting collection.
- How it is used
- Uses guards to exclude selected iterable elements while constructing lists concisely.
- How deep it goes
- Defines truthy inclusion, demonstrates comprehensions and filter(), and shows empty results without examining interpreter internals.
- Examples
- Demonstrates consonant extraction, blue-car selection, numbers greater than ten, and rhyming-prefix exclusion.
- What you can do after
- Apply it via list comprehensions or filter() to build collections containing only elements that satisfy a condition.
Read how Tiny Python Projects: Learn coding and testing with puzzles and games explains guard →
Idiomatic Rust
A match guard is an if condition attached to a pattern that must also be true for the branch to match.
- How it is used
- Uses a guard to add a conditional test to a pattern-matching branch.
- How deep it goes
- Defines the guard and demonstrates matched-value and separate-value conditions in a multi-branch example.
- Examples
- Demonstrates an `i32` value matched alongside a Boolean selector, with guarded branches and a fallback.
- What you can do after
- Apply guards to select match branches using the matched value or another value.
Related concepts
- Haskellused by · Haskell is a statically typed, purely functional programming language with non-strict, typically lazy evaluation.
- conditional logicuses · Conditional logic is a programming technique that selects execution paths, actions, or result values according to evaluated conditions.
- pattern matchingpart of · Pattern matching is the process of comparing data or values with specified or learned patterns to identify correspondence, conformity, or a matching case.
- comprehensionpart of · A comprehension is a language construct that creates a collection by iterating over one or more sources, optionally filtering elements and transforming them.
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