Dictionary patterns

Dictionary patterns require particular keys and match their values at the same time. Use one when a branch cares about a known subset of a lookup:

if lookup is ["name": let name, "active": true] {
    Console.WriteLine(name)
}

The pattern succeeds only when the input is dictionary-compatible, every written key exists, and every nested value pattern succeeds.

Raven supports IDictionary<TKey, TValue>, IReadOnlyDictionary<TKey, TValue>, and types that implement either interface. Each key expression is converted to TKey; its nested pattern is checked against TValue.

Entries are evaluated from left to right. A key is evaluated before its value is read and matched.

Dictionary deconstruction

The same keyed shape can extract values in a declaration or assignment:

let values: IReadOnlyDictionary<string, int> = ["a": 2, "b": 3]
let ["a": first, "b": second] = values

var left = 0
var right = 0
["a": left, "b": right] = values

Deconstruction is extraction-oriented rather than a boolean test. Raven first checks that the source is dictionary-compatible, then reads each requested key and assigns its value through the nested designation or deconstruction pattern.

General dictionary matching supports the same nested pattern vocabulary as other matching contexts. Deconstruction uses the narrower extraction forms described under Matching and deconstruction.