It’s an eloquent testimony to the dangers of dynamic scope that even the very first example of higher-order Lisp functions was broken because of it. It may be that McCarthy was not fully aware of the implications of dynamic scope in 1960. Dynamic scope remained in Lisp implementations for a surprisingly long time—until Sussman and Steele developed Scheme in 1975. Lexical scope does not complicate the definition of eval very much, but it may make compilers harder to write.
