A program transformation to remove free variables. An
expression containing a free variable is replaced by a
function applied to that variable. E.g.
f x = g 3 where g y = y + x
x is a free variable of g so it is added as an extra argument:
f x = g 3 x where g y x = y + x
Functions like this with no free variables are known as
supercombinators and are traditionally given upper-case names
beginning with "$". This transformation tends to produce many
supercombinators of the form f x = g x which can be eliminated
by
eta reduction and substitution. Changing the order of
the parameters may also allow more optimisations. References
to global (top-level) constants and functions are not
transformed to function parameters though they are technically
free variables.
A closely related technique is closure conversion. See also
Full laziness.