Comparisons between Nullable and non-Nullable

[mkborregaard]

I am seeing some strange behaviour from Query.jl (v. 0.3.0) when comparing Nullables to non-Nullables.
e.g.

# This first bit just illustrates that the issues derive from Query
julia> using DataFrames

julia> 3 < Nullable(4)
ERROR: MethodError: no method matching isless(::Int64, ::Nullable{Int64})
Closest candidates are:
  isless(::Nullable{Union{}}, ::Nullable{T}) at /Users/michael/.julia/v0.5/NullableArrays/src/operators.jl:161
  isless(::Real, ::AbstractFloat) at operators.jl:41
  isless(::Real, ::Real) at operators.jl:75
  ...
 in <(::Int64, ::Nullable{Int64}) at ./operators.jl:63

julia> using Query
WARNING: Method definition require(Symbol) in module Base at loading.jl:345 overwritten in module Query at /Users/michael/.julia/v0.5/Requires/src/require.jl:12.
WARNING: Method definition ==(Base.Nullable{#T<:Any}, Base.Nullable{Union{}}) in module NullableArrays at /Users/michael/.julia/v0.5/NullableArrays/src/operators.jl:140 overwritten in module Query at /Users/michael/.julia/v0.5/Query/src/operators.jl:8.
WARNING: Method definition ==(Base.Nullable{Union{}}, Base.Nullable{#T<:Any}) in module NullableArrays at /Users/michael/.julia/v0.5/NullableArrays/src/operators.jl:139 overwritten in module Query at /Users/michael/.julia/v0.5/Query/src/operators.jl:9.
WARNING: Method definition ==(Base.Nullable{#T1<:Any}, Base.Nullable{#T2<:Any}) in module NullableArrays at /Users/michael/.julia/v0.5/NullableArrays/src/operators.jl:130 overwritten in module Query at /Users/michael/.julia/v0.5/Query/src/operators.jl:77.

# This is the behaviour I find strange
julia> 3 < Nullable(4)
true
julia> 3 < Nullable()
false

The scalar boolean return value may be necessary for type stability, but I am thinking this may cause issues (I would expect the last expression to return Null).

[davidanthoff]

The next version of Query will no longer engage in this kind of type piracy, i.e. it will no longer overwrite comparison operators for Nullable. Instead, Query will use a custom type for missing data values within queries (probably called DataValue). For that type, 3 < DataValue() will still return false. For Query to work, predicates need to return Bool values, and that really seems the only way to do that right now.