Programming Flix

Type Match

Flix supports a type match construct that enables compile-time pattern matching on the type of a polymorphic value.

For example, we can write a function that inspects the type of its argument:

def inspect(x: a): String = typematch x {
    case _: Int32   => "x is an Int32"
    case _: String  => "x is a String"
    case _: _       => "x is neither an Int32 nor a String"
}

def main(): Unit \ IO = 
    println(inspect(12345));
    println(inspect("abc"));
    println(inspect(false))

Here the inspect function pattern matches on the type of the formal parameter x using the typematch construct. For example, if the type of x is an Int32 then the function returns the string "x is an Int32" and so forth.

The typematch construct is eliminated at compile-time, hence there is no runtime cost.

As the example shows, the typematch construct always requires a default case. This is because Flix has infinitely many types, and a typematch cannot cover all of them.

A type match can also inspect more complex types, as the following example shows:

def inspect(x: a): String = typematch x {
    case _: List[Int32]   => "x is a list of integers"
    case _: List[String]  => "x is a list of strings"
    case _: _             => "x is something else"
}

def main(): Unit \ IO = 
    println(inspect(1 :: 2 :: 3 :: Nil));
    println(inspect("abc" :: "def" :: Nil));
    println(inspect(false))

We can also bind values with type match, as the following example shows:

def inspect(x: a): String = typematch x {
    case i: Int32   => "${i * i}"
    case s: String  => String.toUpperCase(s)
    case _: _ 		=> "-"
}

def main(): Unit \ IO = 
    println(inspect(12345));
    println(inspect("abc"));
    println(inspect(false))

Warning: While type match is a powerful meta-programming construct, it should be sparingly and with great care.

A typical legitimate use case for type match is when we want to work around limitations imposed by the JVM. For example, the Flix Standard Library use type match to implement the Array.copyOfRange function as shown below:

def copyOfRange(_: Region[r2], b: Int32, e: Int32, a: Array[a, r1]): ... =
typematch a {
    case arr: Array[Int16, r1] =>
        import static java.util.Arrays.copyOfRange(Array[Int16, r1], Int32, Int32): ...
    ...
    case arr: Array[Int32, r1] =>
        import static java.util.Arrays.copyOfRange(Array[Int32, r1], Int32, Int32): ...
    ...
    // ... additional cases ...
}

Here type match allows us to call the right overloaded version of java.util.Arrays.copyOfRange. Thus Flix programmers can use our version of copyOfRange (i.e., Array.copyOfRange) while underneath the hood, we always call the most efficient Java version.