[julia-users] @inbounds is broken?

[vavasis]

Dear Julia users,

The usage of the @inbounds macro is not explained the manual, and its 
syntax appears to be strange.  Consider the three functions at the end of 
this posting.  Only the third one works -- why?

In general, I think @inbounds is broken.  Besides the weird syntax, it has 
two other issues.  First, there is no way to apply the macro to one 
subscript operation but not another in a long expression (as far as I 
know).  Second, it is not extensible in the sense that if programmer A 
implements his/her own array-like structure with his/her own getindex and 
setindex operations, he/she might like to have two versions of 
getindex/setindex, one safe/slower and the other unsafe/faster, but there 
is no way for programmer A to detect whether user B, a user of his/her new 
array-like structure, has requested @inbounds or not.

I would like to propose the following replacement for @inbounds, which 
solves all three problems.  Instead of a macro, there should be two 
different subscript operations, say a[1] and a[$ 1 $], where the first is 
safe/slow and the second is unsafe/fast.  The compiler will compile the 
first as getindex/setindex and the second as getindexUnsafe/setindexUnsafe.

-- Steve Vavasis
 


function sqrtfirst{T}(a::Array{T, 1})
    @assert(size(a,1) >= 1)
    @inbounds sqrt(a[1])
end

function sqrtfirst{T}(a::Array{T, 1})
    @assert(size(a,1) >= 1)
    return @inbounds sqrt(a[1])
end

function sqrtfirst{T}(a::Array{T, 1})
    @assert(size(a,1) >= 1)
    @inbounds return sqrt(a[1])
end