13.10 Fast-Load Serialization

The v argument must be a value that could be quoted as a literal—that is, a value without syntax objects for which (compile `',v) would work and be readable after write—or it can include correlated objects mixed with those values. The byte string produced by s-exp->fasl does not use the same format as compiled code, however.

If a value within v is not valid as a quoted literal, and if handle-fail is not #f, then handle-fail is called on the nested value, and the result of handle-fail is written in that value’s place. The handle-fail procedure might raise an exception instead of returning a replacement value. If handle-fail is #f, then the exn:fail:contract exception is raised when an invalid value is encountered.

If external-lift? is not #f, then it receives each value v-sub encountered in v by s-exp->fasl. If the result of external-lift? on v-sub is true, then v-sub is not encoded in the result, and it instead treated as externally lifted. A deserializing fasl->s-exp receives a external-lifts vector that has one value for each externally lifted value, in the same order as passed to external-lift? on serialization.

Like (compile `',v), s-exp->fasl does not preserve graph structure, support cycles, or handle non-prefab structures. Compose s-exp->fasl with serialize to preserve graph structure, handle cyclic data, and encode serializable structures. The s-exp->fasl and fasl->s-exp functions consult current-write-relative-directory and current-load-relative-directory, respectively, in the same way as bytecode saving and loading to store paths in relative form, and they similarly allow and convert constrained srcloc values (see Printing Compiled Code).

Unless keep-mutable? is provided as true to s-exp->fasl, then mutable values in v are replaced by immutable values when the result is decoded by fasl->s-exp. Unless datum-intern? is provided as #f, then any immutable value produced by fasl->s-exp is filtered by datum-intern-literal. The defaults make the composition of s-exp->fasl and fasl->s-exp behave like the composition of write and read.

If skip-prefix? is not #f, then a prefix that identifies the stream as a serialization is not written by s-exp->fasl or read by fasl->s-exp. Omitting a prefix can save a small amount of space, which can useful when serializing small values, but it gives up a sanity check on the fasl->s-exp that is often useful.

The byte-string encoding produced by s-exp->fasl is independent of the Racket version, except as future Racket versions introduce extensions that are not currently recognized. In particular, the result of s-exp->fasl will be valid as input to any future version of fasl->s-exp (as long as the skip-prefix? arguments are consistent).

Changed in version 6.90.0.21 of package base: Made s-exp->fasl format version-independent and added the #:keep-mutable? and #:datum-intern? arguments.
Changed in version 7.3.0.7: Added support for correlated objects.
Changed in version 7.5.0.3: Added the #:handle-fail argument.
Changed in version 7.5.0.9: Added the #:external-lift? and #:external-lifts arguments.