5.1.24. Pattern Matching

This tutorial covers daslang’s pattern matching with the match macro from daslib/match.

5.1.24.1. Basic match

match(expr) { ... } dispatches on values. Each arm is an if:

require daslib/match

def describe(n : int) : string {
    match (n) {
        if (0) { return "zero" }
        if (1) { return "one" }
        if (_) { return "other" }
    }
    return "unreachable"
}

Note

match works on functions that return values.

5.1.24.2. Wildcards and binding

_ — wildcard, matches anything

$v(name) — bind matched value to a variable:

match (point) {
    if (Point(x = 0, y = 0)) { return "origin" }
    if (Point(x = $v(x), y = $v(y))) { return "({x}, {y})" }
}

5.1.24.3. Guards

Add && after a pattern for extra conditions:

match (shape) {
    if (Shape(size = $v(sz)) && sz > 100) {
        return "large"
    }
}

5.1.24.4. OR patterns

Match multiple alternatives with ||:

match (color) {
    if (Color.red || Color.green || Color.blue) {
        return true
    }
}

5.1.24.5. Variant matching

Use $v(name) as alternative to match variant alternatives:

match (value) {
    if ($v(i) as i) { return "int" }
    if ($v(f) as f) { return "float" }
}

5.1.24.6. Tuple matching

Match tuple elements positionally:

match (pair) {
    if ((0, "zero")) { return "exact" }
    if ((1, _)) { return "starts with one" }
    if (($v(n), "hello")) { return "hello #{n}" }
}

5.1.24.7. Array matching

Static arrays match element-by-element. Dynamic arrays match head elements; use ... to ignore the tail:

match (sa) {
    if (fixed_array<int>(0, 0, 0)) { return "zeros" }
    if (fixed_array<int>(1, $v(b), $v(c))) { return "1,{b},{c}" }
}

match (da) {
    if (array<int>(0, 0, ...)) { return "starts with 0,0" }
    if (array<int>($v(x), $v(y), ...)) { return "starts with {x},{y}" }
}

5.1.24.8. match_expr — computed patterns

match_expr(expression) evaluates at runtime instead of matching literally. Useful when the expected value depends on a captured variable:

match (t) {
    if (($v(a), match_expr(a + 1), match_expr(a + 2))) {
        return true   // matches consecutive triples
    }
}

5.1.24.9. Bool matching

match works on booleans:

match (b) {
    if (true) { return "yes" }
    if (false) { return "no" }
}

5.1.24.10. multi_match and static_match

multi_match — all matching arms execute (not just first)
static_match — silently drops type-mismatched arms (for generics)

multi_match (n) {
    if ($v(a) && a > 0) { tags += " positive" }
    if ($v(a) && (a % 2 == 0)) { tags += " even" }
}