We can define functions to factor out a repeated piece of logic that we later reuse multiple times in a design. For example, we can wrap up our earlier example of a simple combinational logic block as follows:

def clb(a: UInt, b: UInt, c: UInt, d: UInt): UInt =
  (a & b) | (~c & d)

where clb is the function which takes a, b, c, d as arguments and returns a wire to the output of a boolean circuit. The def keyword is part of Scala and introduces a function definition, with each argument followed by a colon then its type, and the function return type given after the colon following the argument list. The equals (=})sign separates the function argument list from the function definition.

We can then use the block in another circuit as follows:

val out = clb(a,b,c,d)

We will later describe many powerful ways to use functions to construct hardware using Scala’s functional programming support.