260: Generated Symbols260: Generated Symbolsby Marc Nieper-Wißkirchen
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This SRFI defines the procedure generate-symbol. Each
time it is invoked, the procedure returns a new symbol whose name
cannot be guessed. The returned symbol is a standard symbol for all
purposes; it obeys write/read invariance and it is equal to another
symbol if and only if their names are spelt the same.
None at present.
A unique object is a Scheme value that is not equal
(usually with respect to eqv?) to existing (or potentially
existing) objects. Unique objects have many use cases, for example as
private keys in data structures or to uniquely mark other values.
Scheme has many ways to create such unique objects. For many
purposes, an expression like (string #\m) can be used to
create a unique object. To improve type safety, alternatively, a
constructor of a record type with at least one mutable field can be
invoked.
There are circumstances where such unique objects, may they be strings or records, cannot be used because an object of a particular type, specifically a symbol, is needed. In the case of Scheme, this applies to its syntax system. An identifier is a syntax object that is a wrapped symbol (see R6RS). To generate unique identifiers with a given wrap, unique symbols (and not strings, records, etc.), therefore, have to be generated.
The procedure generate-symbol defined in this SRFI
provides a means to generate unique symbols.
Alternative proposals
like SRFI
258 provide so-called uninterned symbols that can also be used for
this purpose (at least after amending the definition of a syntax
object). Introducing uninterned symbols into the standard, however,
would weaken the semantics of symbols. Two symbols would no longer be
identical (in the sense of eqv?) if and only if their
names are spelt the same way.
Uninterned symbols have another disadvantage in that they defeat write/read invariance. Yet, some kind of write/read invariance is likely needed when uninterned symbols are used in identifier names so that expanded Scheme code can be serialised (e.g. for separate AOT expansion/compilation).
As names for generated symbols can be generated lazily, the alleged
advantage of uninterned symbols — that no names have to be generated — is
a bogus one. A name for a generated symbol only has to be generated
for use cases that need the unique name
from symbol->string, e.g. for write/read purposes, but
these use cases are not covered by uninterned symbols anyway.
Generated symbols need no new lexical syntax. The notion of a
Scheme datum value does not change; a Scheme datum value remains a
Scheme value that obeys write/read equivalence (up
to equal?).
The following binding is exported by the (srfi :260 generate-symbol) library:
Procedure: generate-symbol [pretty-name]
The optional argument, pretty-name, must, if present, be a string.
Returns a symbol that is not identical to any existing or potentially existing symbol, for all practical purposes.
Note: This means that for every Scheme
expression ⟨expression⟩ whose (possibly
non-deterministic) value does not depend on the result
of symbol->string applied to the
result gensym of a previous invocation
of generate-symbol, its value is not equal (in the sense
of equal?) to the value
of (symbol->string gensym) for
all practical purposes.
The implementation may use the argument pretty-name for debugging or pretty-printing purposes.
(let ([s (generate-symbol)]) (symbol=? s s)) ; => #t
(let ([s1 (generate-symbol)] [s2 (generate-symbol)]) (symbol=? s1 s2)) ;=> #f
(let ([s1 (generate-symbol "g1")] [s2 (generate-symbol "g1")]) (symbol=? s1 s2)) ;=> #f
(let ([s1 (generate-symbol "g1")] [s2 'g1]) (symbol=? s1 s2)) ;=> #f
(let ([s (generate-symbol "g1")]) (string=? (symbol->string s) "g1")) ;=> #f
(let ([s (generate-symbol)]) (symbol=? (string->symbol (symbol->string s)) s)) ;=> #t
A native implementation does not need to create a name for the
symbol returned by an application of generate-symbol
immediately but only lazily when symbol->string is
applied to the symbol for the first
time. If symbol->string is never applied as will likely
be the case in many uses of generated symbols, no name has to be
created. Symbols generated by generate-symbol can
therefore replace uninterned symbols known from some Scheme
implementations.
When an implementation has to create a name, it can simply choose a
randomly generated name so that the entropy of the random variable
described by (lambda () (symbol->string (generate-symbol))) is
sufficiently high (e.g. 122 bits as in the case of Version 4 UUIDs but
note that this SRFI prescribes no particular form of the symbol name).
The portable sample
implementation creates random names eagerly and
uses /dev/random for this purpose.
This SRFI was created in response to SRFI 258: Uninterned Symbols.
© 2025 Marc Nieper-Wißkirchen.
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