Sometimes one would like to create a closure directly, without bothering to give it a name. There is a special keyword lambda for this purpose. By writing a function definition without the keyword def and replacing the function name with the keyword lambda, that definition becomes an expression which publishes a closure.

Note that a lambda cannot create a recursive function, since the function is not given a name in the body.

{- Define a function that sums the results of its argument function
   evaluated with arguments 1 and 2
-}

def onetwosum(f) = f(1) + f(2)

onetwosum( lambda(x) = x * 3 )

{- 
  identical to:
  def triple(x) = x * 3
  onetwosum(triple)
-}

{-
OUTPUT:
9
-}