[julia-users] Type stability of map - is there a better way to do this?

[Tomas Lycken]

I have a `DataArray{Float64,1}` named `ps` with a bunch of values I'd like 
to map to their bin indices in a histogram, and a function `histidx` that 
maps a single value (based on endpoints of the histogram and number of 
bins). I try the following:

idxs = map(p -> histidx(minx, maxx, Nbins, p), ps)

and the result is a `DataArray{Any,1}`. However, I know that `histidx` can 
only return integers:

function histidx(xmin, xmax, Nbins::Integer, x)
    if x <= xmin
        return 1
    elseif x >= xmax
        return Nbins
    else
        return iceil((x-xmin)/((xmax-xmin)/Nbins))
    end
end

Why isn't map able to infer that the type of the resulting array could be 
tighter?

I try to help it in various ways, but without success:

julia> typeof(map(p -> histidx(minx, maxx, Nbins, p)::Integer, ps))
DataArray{Any,1}
julia> foo() = map(p -> histidx(minx, maxx, Nbins, p); typeof(foo())
DataArray{Any,1}

The only thing that seems to work is manually doing this on the underlying 
data instead:

julia> typeof(map(p -> histidx(minx, maxx, Nbins, p), ps.data))
Array{Int64,1}

This is troublesome, since it a) requires me to handle `Array`s and 
`DataArray`s differently when I do this mapping, and b) doesn't allow for 
missing data in the `DataArray`s.

I think the correct behavior should be to return a DataArray{Int64,1}, with 
each `NA` value mapped to `NA` in the output.

// T