Re: Guile: What's wrong with this?

[David Kastrup]

Mike Gran <address@hidden> 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