What are valid identifiers in R7RS-small?

Question

R7RS-small says that all identifiers must be terminated by a delimiter, but at the same time it defines pretty elaborate rules for what can be in an identifier. So, which one is it?

Is an identifier supposed to start with an initial character and then continue until a delimiter, or does it start with an initial character and continue following the syntax defined in 7.1.1.

Here are a couple of obvious cases. Are these valid identifiers?

• a#a
• b,b
• c'c
• d[d]

If they are not supposed to be valid, what is the purpose of saying that an identifier must be terminated by a delimiter?

Answer 1

|..ident..| are delimiters for symbols in R7RS, to allow any character that you cannot insert in an old style symbol (| is the delimiter).

However, in R6RS the "official" grammar was incorrect, as it did not allow to define symbols such that 1+, which led all implementations define their own rules to overcome this illness of the official grammar.

Unless you need to read the source code of a given implementation and see how it defines the symbols, you should not care too much about these rules and use classical symbols.

In the section 7.1.1 you find the backus-naur form that defines the lexical structure of R7RS identifiers but I doubt the implementations follow it.

I quote from here

As with identifiers, different implementations of Scheme use slightly different rules, but it is always the case that a sequence of characters that contains no special characters and begins with a character that cannot begin a number is taken to be a symbol

In other words, an implementation will use a function like read-atom and after that it will classify an atom by backtracking with read-number and if number? fails it will be a symbol.