Equality of FunctionTerms (#205)

* confirm failing test for #204

* equality for FuncionTerm check function and original expression

* test that adding two non-equal FunctionTerms works correctly

src/terms.jl

@@ -127,6 +127,8 @@ FunctionTerm(forig::Fo, fanon::Fa, names::NTuple{N,Symbol},
     FunctionTerm{Fo, Fa, names}(forig, fanon, exorig, args_parsed)
 width(::FunctionTerm) = 1
 
+Base.:(==)(a::FunctionTerm, b::FunctionTerm) = a.forig == b.forig && a.exorig == b.exorig
+
 """
     InteractionTerm{Ts} <: AbstractTerm
 

test/terms.jl

@@ -86,6 +86,18 @@ StatsModels.apply_schema(mt::MultiTerm, sch::StatsModels.Schema, Mod::Type) =
         @test ab+bc == abc
     end
 
+    @testset "uniqueness of FunctionTerms" begin
+        f1 = @formula(y ~ lag(x,1) + lag(x,1))
+        f2 = @formula(y ~ lag(x,1))
+        f3 = @formula(y ~ lag(x,1) + lag(x,2))
+
+        @test f1.rhs == f2.rhs
+        @test f1.rhs != f3.rhs
+
+        ## addition of two identical function terms
+        @test f2.rhs + f2.rhs == f2.rhs
+    end
+
     @testset "expand nested tuples of terms during apply_schema" begin
         sch = schema((a=rand(10), b=rand(10), c=rand(10)))