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
