Generic Functions
This section deals with the generic functions; defun, let, lambda, eval-kl, freeze and thaw.
defun
'defun' in Kλ requires little explanation except to note that 'defun' is a top level construction and there
is no obligation to support the evaluation of 'defun' within an expression; this also holds true of 'define' unless
it is used in a package.
Before the S series of kernels, it was mandatory that all functions defined by 'defun' sustain currying and
hence partial applications. This is now handled by the Shen reader and not Kλ.
lambda
'lambda' in Kλ is deliberately spartan; following lambda calculus in defining an abstraction that
accepts only one argument. let is strictly otiose being definable by the equation.
(let X Y Z) = ((lambda X Z) Y)
However this form is less natural and less familiar than the traditional local assignment and is not
definable except by a macro. Note that in Shen '(lambda X X)' is legal.
eval-kl
'eval-kl' evaluates a Kλ expression. It is generally not used within applications programming.
The function 'eval' is not a primitive in Kλ, but 'eval-kl' is.
'eval' in Shen applied to an an expression E returns the expression E' that results from evaluating the
expression E'' where E'' results from E by the replacement of all the square brackets in E' by round ones. Thus
'(eval [+ 1 2])' evaluates to what '(+ 1 2) 'evaluates to - which is 3.
freeze and thaw
The function 'freeze' freezes the computation represented by its argument which is not evaluated. Effectively
'freeze' returns a continuation; defined by Wikipedia as "[the reification of] an instance of a computational process
at a given point in the process's execution". The counterpart to 'freeze' is the function 'thaw' which unfreezes the
computation and returns the evaluated result. 'thaw' is not primitive being readily defined in Kλ as
(defun thaw (F) (F)) |
