tds.Vec.sort fails in LUA52 & LUA53 with conversion error

When changing to tds as a data backend for the torch-dataframe I noticed that the sort for tds.Vec fails in LUA52 & LUA53‡ environments with the error:

unable to convert argument 2 from cdata<int ()(struct tdselem, struct tdselem)> to cdata<int ()(const struct tdselem, const struct tdselem)> stack traceback: /opt/torch/install/share/lua/5.2/trepl/init.lua:501: in function </opt/torch/install/share/lua/5.2/trepl/init.lua:494> [C]: in function 'tds_vec_sort' /opt/torch/install/share/lua/5.2/tds/vec.lua:98: in function 'sort' [string "_RESULT={b:sort(function(a,b) return a < b en..."]:1: in main chunk [C]: in function 'xpcall' /opt/torch/install/share/lua/5.2/trepl/init.lua:652: in function 'repl' /opt/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:199: in main chunk [C]: in ?

Here's code that runs fine in standard LUAJIT:

th> tds = require 'tds'
                                                                      [0.0014s]
th> a = tds.Vec("ba", "ab", "c", "AA")
                                                                      [0.0001s]
th> a:sort(function(a, b) return string.lower(a) < string.lower(b) end)  
                                                                      [0.0001s]
th> a
tds.Vec[4]{
    1 : AA
    2 : ab
    3 : ba
    4 : c
}
                                                                      [0.0001s]
th> a:sort(function(a, b) return a:lower() < b:lower() end)  
                                                                      [0.0001s]
th> a
tds.Vec[4]{
    1 : AA
    2 : ab
    3 : ba
    4 : c
}
                                                                      [0.0001s]
th> a:sort(function(a, b) return a:lower() > b:lower() end)  
                                                                      [0.0003s]
th> a
tds.Vec[4]{
    1 : c
    2 : ba
    3 : ab
    4 : AA
}
                                                                      [0.0001s]
th> b = tds.Vec({2,1,3,0})
                                                                      [0.0001s]
th>  b:sort(function(a,b) return a < b end)
                                                                      [0.0001s]
th> b
tds.Vec[4]{
    1 : 0
    2 : 1
    3 : 2
    4 : 3
}

The same code in a LUA52 Docker container results in:

...
th> a:sort(function(a, b) return a:lower() > b:lower() end)
unable to convert argument 2 from cdata<int (*)(struct tds_elem_*, struct tds_elem_*)> to cdata<int (*)(const struct tds_elem_*, const struct tds_elem_*)>
stack traceback:
    /opt/torch/install/share/lua/5.2/trepl/init.lua:501: in function </opt/torch/install/share/lua/5.2/trepl/init.lua:494>
    [C]: in function 'tds_vec_sort'
    /opt/torch/install/share/lua/5.2/tds/vec.lua:98: in function 'sort'
    [string "_RESULT={a:sort(function(a, b) return a:lower..."]:1: in main chunk
    [C]: in function 'xpcall'
    /opt/torch/install/share/lua/5.2/trepl/init.lua:652: in function 'repl'
    /opt/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:199: in main chunk
    [C]: in ?   
                                                                      [0.0005s]
...
th> b:sort(function(a,b) return a < b end)
unable to convert argument 2 from cdata<int (*)(struct tds_elem_*, struct tds_elem_*)> to cdata<int (*)(const struct tds_elem_*, const struct tds_elem_*)>
stack traceback:
    /opt/torch/install/share/lua/5.2/trepl/init.lua:501: in function </opt/torch/install/share/lua/5.2/trepl/init.lua:494>
    [C]: in function 'tds_vec_sort'
    /opt/torch/install/share/lua/5.2/tds/vec.lua:98: in function 'sort'
    [string "_RESULT={b:sort(function(a,b) return a < b en..."]:1: in main chunk
    [C]: in function 'xpcall'
    /opt/torch/install/share/lua/5.2/trepl/init.lua:652: in function 'repl'
    /opt/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:199: in main chunk
    [C]: in ?

‡ probably also allpies to LUA51 but this hasn't been tested as we've dropped 51 support

torch / tds

tds.Vec.sort fails in LUA52 & LUA53 with conversion error #25