A collection associating keys with values and supporting lookup by key. Unlike an ordered list of guarded alternatives, a simple map does not directly represent sequentially evaluated, potentially repeated label expressions with first-match behavior.
An associative array represented by a list of key-value pairs. Looking up a key scans entries until a matching key is found, giving linear worst-case search cost in the number of entries when key comparisons have unit cost.
Represent a total function by storing only selected entries of a finite partial function and returning a fixed default for missing keys. A missing payment-count record can denote two zero counts. Storing an explicit default-valued record can have the same observable meaning, so the abstraction function need not be injective. Update operations must preserve all unrelated keys and establish the intended change whether or not the key was previously stored.
