Can not use CuArray on Julia 0.7

When I use 0.7 and try to use CuArray, the following error happens:

julia> using CuArrays
┌ Warning: `using A: B` will only be allowed for single bindings, not modules. Use `using A.B` instead
│   caller = ip:0x0
└ @ Core :-1
WARNING: could not import Base.showarray into GPUArrays
WARNING: could not import GPUArrays.allowscalar into CuArrays
WARNING: could not import GPUArrays.@allowscalar into CuArrays
ERROR: LoadError: LoadError: UndefVarError: GPUBackend not defined
 [1] getproperty(::Module, ::Symbol) at ./sysimg.jl:13
 [2] top-level scope at none:0
 [3] include at ./boot.jl:317 [inlined]
 [4] include_relative(::Module, ::String) at ./loading.jl:1038
 [5] include at ./sysimg.jl:29 [inlined]
 [6] include(::String) at /home/xhu/.julia/packages/CuArrays/clDeS/src/CuArrays.jl:3
 [7] top-level scope at none:0
 [8] include at ./boot.jl:317 [inlined]
 [9] include_relative(::Module, ::String) at ./loading.jl:1038
 [10] _require(::Base.PkgId) at ./loading.jl:950
 [11] require(::Base.PkgId) at ./loading.jl:852
 [12] macro expansion at ./logging.jl:311 [inlined]
 [13] require(::Module, ::Symbol) at ./loading.jl:834
in expression starting at /home/xhu/.julia/packages/CuArrays/clDeS/src/gpuarray_interface.jl:3
in expression starting at /home/xhu/.julia/packages/CuArrays/clDeS/src/CuArrays.jl:38

Does this mean that GPUArrays are not imported?

Package versions? CuArrays is working fine on 0.7:

Through trail and error, I found out that I have 3 versions of GPUArrays, 2 in ~/.Julia/packages, 1 in ~/.Julia/dev. Once I removed all three and added the GPUArray packages again, which placed the package in ~/.Julia/dev, and it worked fine. So, does this mean, right now it is best to use the dev version of GPUArray? And also a side question, what determines packages store location (that is, in ~/.Julia/packages or ~/.Julia/dev)?

No. the tagged and released versions should work just fine. You should only need to dev packages if you want to contribute to them. If you need the master branch, e.g. because a bugfix, it’s better to add MyPackage#master.

Whether you add or dev. The exact location is controlled by environment variables (JULIA_DEPOT_PATH and JULIA_PKG_DEVDIR).