Mike Gran <spk...@yahoo.com> writes: >> In many systems it is desirable for constants (i.e. the values of literal >> expressions) to reside in read-only-memory. To express this, it is >> convenient to imagine that every object that denotes locations is >> associated with a flag telling whether that object is mutable or immutable. >> In such systems literal constants and the strings returned by >> `symbol->string' are immutable objects, while all objects created by >> the other procedures listed in this report are mutable. It is an error >> to attempt to store a new value into a location that is denoted by an >> immutable object. >> >> In Guile this has been the case since commit >> 190d4b0d93599e5b58e773dc6375054c3a6e3dbf. >> >> The reason for this is that Guile’s compiler tries hard to avoid >> duplicating constants in the output bytecode. Thus, modifying a >> constant would actually change all other occurrences of that constant in >> the code, making it a non-constant. ;-) > > This is a terrible example of the RnRS promoting some strange idea of > mathematical purity over being useful. > > The idea that the correct way to initialize a string is > (define x (string-copy "string")) is awkward. "string" is a read-only > but copying it makes it modifyiable? Copying implies mutability? > > Copying doesn't imply modifying mutability in any other data type.
Huh? (set-car! '(4 5) 3) => bad (set-car! (list-copy '(4 5)) 3) => ok Similar with literal vectors. Why should strings be different here? -- David Kastrup
