Tuples -- who needs 'em

Jonathan Giddy jon at dgs.monash.edu.au
Thu Apr 6 03:37:07 CEST 2000


I nearly fell out of my tree when Alexander, Bob declared:

>I am wondering where the serious concern about mutable lists comes from, as
>it is a pattern in the responses I've gotten on this topic. For me, the
>immutable list is a relatively new concept, since I can't think of many
>other languages that have it. I'm a programming-language-design kind of
>person, and I never really saw mutable lists as a problem that needed
>solving. 

How about C?  Surely 'const' is an attempt to mark arrays as immutable.
Apparently, the ANSI C committee saw mutable arrays as a problem that
needed solving.

>I've used many Python-type languages that have mutable lists but
>not immutable ones, and if I don't want the elements to change they don't
>change -- I simply avoid the x[i] = y construct :-) 

Avoiding the x[i] = y construct is much harder in non-trivial programs, 
where assumptions about mutability need to be specified as part of the call
interface.  Hence the proliferation of const in the C string library.

>And I still haven't
>heard from anyone about why there isn't similar concern about immutable
>dictionaries.

I could express some concern, but dictionaries are used much less than
sequences, and of all the places in my code where I make possibly needless
copies of a dictionary, it is usually to modify the dictionary with a clear
conscience, not something that immutability would solve.

Jon.




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