Lambda lifting is a program transformation to remove free variables where an expression containing a free variable is replaced by a function applied to that variable. This is useful for the compilation of programs in a FunctionalProgrammingLanguage because it allows you to transform a functional program with local function definitions into a program consisting only of global function definitions. The reverse transformation is called LambdaDropping.
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An example:

Here is a function '''sum''' that returns the sum of two variables, '''x''' and '''y''', which are defined outside the function:
 (defun sum () (+ x y))
Note that, since neither '''x''' nor '''y''' is a parameter of '''sum''', each is a FreeVariable in '''sum'''.

If we apply LambdaLifting to '''sum''', we get a slightly different function:
 (defun sum (x y) (+ x y))
Now, since '''x''' and '''y''' are parameters of '''sum''', they are no longer free variables in it.

Of course, all instances of '''sum''' in the original program now need to supply '''x''' and '''y''' as arguments:
 (sum)
becomes:
 (sum x y)

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Lambda lifting is somewhat similar to the MoveField-refactoring used to transform programs in an ObjectOrientedProgrammingLanguage.
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CategoryRefactoring