letrec

Schemeでは次のような式はエラーになることを初めて知った。

(let ((x (+ x 1))) x)

(+ x 1) の x が未定義になるため。
R5RSによると，letrecの意味は

library syntax: (letrec )

Syntax: should have the form

(( ) ...),

and should be a sequence of one or more expressions. It is an error for a to appear more than once in the list of variables being bound.

Semantics: The s are bound to fresh locations holding undefined values, the s are evaluated in the resulting environment (in some unspecified order), each is assigned to the result of the corresponding , the is evaluated in the resulting environment, and the value(s) of the last expression in is(are) returned. Each binding of a has the entire letrec expression as its region, making it possible to define mutually recursive procedures.

Revised^5 Report on the Algorithmic Language Scheme

となっている。