Iff + When Swift implementation

if+when.swift

typealias IfResultClosure<Result> = (() -> Result)
typealias Case<Value, Result> = (Value, IfResultClosure<Result>)

@_functionBuilder
struct CaseBuilder<Value, Result> {
    static func buildBlock(_ cases: Case<Value, Result>...) -> [Case<Value, Result>] {
        cases
    }
}

func If<Result>(_ predicate: Bool, then: IfResultClosure<Result>, `else`: IfResultClosure<Result?> = { nil }) -> Result? {
    predicate ? then() : `else`()
}

func when<Value : Equatable, Result>(
    _ predicate: Value,
    @CaseBuilder<Value, Result> _ cases: () -> [Case<Value, Result>],
    defaultValue: () -> Result
) -> Result {
    cases().first { $0.0 == predicate }?.1() ?? defaultValue()
}

let whenResult = when("123") {
    Case("1") {
        "result 1"
    }
    Case("12") {
        "result 12"
    }
    Case("123") {
        "result 123"
    }
} defaultValue: {
    "My default value"
}

let isTrue = If(true) {
    "something when true"
} else: {
    "something when false"
}