Term definition

A term definition has the form f p_1 p_2 … p_n = e where f is the name of the term being defined. The parameters p_1 through p_n are the names of parameters, if any (if the term is a function), separated by spaces. The right-hand side of the = sign is any Unison expression.

The names of the parameters as well as the name of the term are bound as local variables in the expression on the right-hand side (also known as the body of the function). When the function is called, the parameter names are bound to any arguments passed in the call. See function application for details on the call semantics of functions.

If the term takes no arguments, the term has the value of the fully evaluated expression on the right-hand side and is not a function.

The expression comprising the right-hand side can refer to the name given to the definition in the left-hand side. In that case, it’s a recursive definition. For example:

sumUpTo : Nat -> Nat

sumUpTo : Nat -> Nat
sumUpTo n =
  use Nat + - <
  if n < 2 then n else n + sumUpTo (n - 1)

The above defines a function sumUpTo that recursively sums all the natural numbers less than some number n. As an example, sumUpTo 3 is 1 + 2 + 3, which is 6.

Note: The expression n Nat.- 1 on line 4 above subtracts 1 from the natural number n. Since the natural numbers are not closed under subtraction (''n Nat.subtractInt 1'' is an Int), we use the operation - which has the convention that 0 - n Nat.== 0 for all natural numbers n. Unison's type system saves us from having to deal with negative numbers here.