Check whether a symbol is bound

Question

In Emacs Lisp (boundp 'symbol) returns t if symbol is bound to some value, nil otherwise. Is there an equivalent procedure in Guile Scheme?

Answer 1

Scheme avoids leaking implementation into the specification and speaks of 'identifiers' rather than of binding an interned symbol to a value - see §2.1 of R7RS. In scheme, an 'identifier' is just a name.

An identifier name is treated as identifying a variable unless it identifies a macro (syntax) or it is in a context requiring it to be treated as identifying a symbol, such as by quotation. In particular, §2.1 of R7RS states that "When an identifier appears as a literal or within a literal (see section 4.1.2), it is being used to denote a symbol (see section 6.5)". You can test whether an identifer identifies a symbol with the symbol? procedure.

Guile scheme does in fact implement identifiers by interning symbols and you can query whether a symbol is bound using defined?:

(defined? 'num)

=> #f

(define num 1)(defined? 'num)

=> #t

This is a guile implementation matter and not portable scheme.

Edit: Note that defined? only works with top level variables defined with define. It does not work with let and cognates.