Even C++ doesn't do nested constness. Hence you have double const * double * const double const * const double * const * const ... all meaning different things.
--Tim On Friday, August 15, 2014 10:26:48 AM Tamas Papp wrote: > Suppose you have an array of arrays (or any other kind of > containers). If I understand correctly, even if I wrap the outer array > as read-only, your proposal would still allow changing the elements in > the inner arrays. Given this, I wonder if your proposal would be a > solution to your problem (I have to admit that I don't have a very clear > understanding of the problem that this solves, but maybe I missed > something in the original thread), and if not, whether the marginal > benefit it is worth the extra complication. > > IMO enforcing that something is read-only is very difficult in languages > that are not functional (eg Haskell) but allow complicated data types. > > Best, > > Tamas > > On Thu, Aug 14 2014, [email protected] wrote: > > Dear Julia colleagues, > > > > In June, I wondered on this newsgroup why Julia lacks an 'inarg' > > specification for functions. The two major languages used nowadays for > > scientific programming, namely Fortran and C++, both provide mechanisms to > > declare that a function argument is read-only by the function. Indeed, > > this was added to Fortran late in its life, so one assumes that there is > > some bitter experience underlying the decision to include it in Fortran. > > > > (Matlab has only inargs, at least until you get to relatively advanced > > > > programming techniques, so this is not an issue.) > > > > Stefan Karpinski provided a convincing explanation for why inarg and > > outarg > > specifications have been omitted from the Julia core. Now that I am > > slightly more familiar with Julia, I would like to make a proposal for > > inarg that would not require changes to the core language. The proposal > > would, however, entail substantial additional code, so it is intended for > > some version of Julia in the future. > > > > Here is the proposal: suppose fn is a function that takes an array input > > "a" and wants to declare it to be read-only. Then at the beginning of the > > function before any of the arguments gets used, there would be an > > assignment statement like this: > > > > function fn(a::AbstractArray{Int,1}) > > a = readonly(a) > > > > The way this would work is as follows: Julia would have new types, mostly > > invisible to the user, like ReadOnlyArray which would have a setindex! > > method that would throw an error, Then there would be methods like > > readonly{T.N}(a::Array{T,N}) that would use 'reinterpret' to change the > > Array to a ReadOnlyArray. A programmer who wanted to write a function > > that > > could take either arrays or readonlyarrays as inputs would have to declare > > abstract argument types like DenseArray or AbstractArray. The user would > > not, in general, have to worry about the read-only types; the situation > > when a user would have to worry about them is if he/she wants to develop a > > read-only version of his/her own data structure that internally uses the > > standard containers. > > > > This proposal has several advantages: > > > > (1) The above mechanism enforces the read-only property of a since fn no > > longer has access to the initial (writeable) definition of a. > > > > (2) The above mechanism, if adopted as an idiom, could be easily spotted > > by > > program analysis tools that could then make optimizations based on the > > fact > > that a is read-only. > > > > (3) There is hardly any performance penalty for this mechanism. > > > > But obviously there is one big disadvantage: > > > > (1) For each of the standard containers, a new read-only version would > > have > > to be written. Furthermore, there would also have to be an abstract type > > to serve as a common parent. E.g. there is currently no abstract parent > > for Set (although there is an open discussion about that matter on > > github). > > > > and a second minor disadvantage > > > > (2) The enforcement of read-only would probably would not work > > recursively, > > e.g., if a were an array of arrays. In fact C++ has a similar gap in its > > 'const' mechanism. > > > > -- Steve Vavasis
