Pattern Matching

Nebra provides match for pattern matching on values.

Match Statement

match status
case "ok" then
    print("Success")
case "error" then
    print("Failure")
case _ then
    print("Unknown")
end

Compiles to an if/elseif/else chain.

Value Patterns

Match against specific values using ==:

match x
case 1 then
    print("one")
case 2 then
    print("two")
case 3 then
    print("three")
case _ then
    print("other")
end

Wildcard Pattern

_ matches anything (catch-all):

match value
case _ then
    print("always matches")
end

Type Binding Pattern

Bind a name to the matched value with a type check:

match value
case s: string then
    print("String: " .. s)
case n: number then
    print("Number: " .. tostring(n))
case _ then
    print("Something else")
end

This checks type(value) == "string" and binds s = value in the arm body.

Guard Expressions

Add an additional condition with when:

match x
case n: number when n > 0 then
    print("Positive: " .. n)
case n: number when n < 0 then
    print("Negative: " .. n)
case n: number then
    print("Zero")
case _ then
    print("Not a number")
end

Match Expression

match can be used as an expression that returns a value:

local label = match code
    case 200 then "OK"
    case 404 then "Not Found"
    case 500 then "Server Error"
    case _ then "Unknown"
end

Match expressions also support guards and type bindings.

Exhaustive Matching

Configure exhaustive matching in nebra.toml:

[rules]
exhaustive_match = "explicit"  # none, relaxed, explicit

• none -- no exhaustiveness check
• relaxed -- warn if wildcard _ is missing
• explicit -- error if not all enum variants are covered (when matching on an enum)

Enum Matching

enum Direction
    North
    South
    East
    West
end

match dir
case Direction.North then print("Going north")
case Direction.South then print("Going south")
case Direction.East then print("Going east")
case Direction.West then print("Going west")
end

With exhaustive_match = "explicit", omitting a variant is a compile error.