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Functional Programming in Scheme
The Order of Evaluation
The alpha rewrite rule

An alpha conversion changes the names of lambda expression formal parameters

Formal parameters of a lambda expression can be substituted by other names, which are not used as free names in the body

Legal conversion:

Expression

Converted Expression

(lambda (x y) (f x y))
(lambda (a b) (f a b))

An example of an alpha rewriting. The name a replaces x and the name b replaces y.

Illegal conversion:

Expression

Converted Expression

(lambda (x y) (f x y))
(lambda (a f) (f a f))

Examples of an illegal alpha conversion. f is a free name in the lambda expression. A free name is used, but not defined (bound) in the lambda expression. In case we rename one of the parameters to f, the free name will be bound, hereby causing an erroneous name binding.