问题
To put it simply: My question is whats is the type of a define expression in Scheme?
Take for example:
(define x 5)
or
(define x (lambda (n) (* n n)))
It's a bit confusing for me. Can anyone help?
回答1:
In Racket define is a special form and not an expression, so it doesn't have a value per-se, if you try to execute something like this you'll get an error:
(display (define x 42))
=> define: not allowed in an expression context in: (define x 42)
If it were to have a value it'd be something akin to void, but that will be dependent on the particular implementation details of the interpreter (I believe I saw one interpreter return #t after a define was completed)
The constant
#<void>is returned by most forms and procedures that have a side-effect and no useful result
The specification doesn't go into details on this point, either - reinforcing the statement that it's implementation-dependent.
来源：https://stackoverflow.com/questions/15504117/type-of-define-expression-in-scheme