Implement simplify(:(x - (-y))) ==> x + y

Author: magistere

src/symbolic.jl

@@ -145,6 +145,9 @@ function simplify(::SymbolParameter{:+}, args)
     end
 end
 
+isminus(ex::Expr) = ex.head == :call && ex.args[1] == :- && length(ex.args) == 2
+isminus(ex) = false
+
 # Assume length(args) == 3
 function simplify(::SymbolParameter{:-}, args)
     # Remove any 0's in a subtraction
@@ -154,6 +157,9 @@ function simplify(::SymbolParameter{:-}, args)
     # Special Case: simplify(:(x - x)) == 0
     elseif length(args) == 2 && args[1] == args[2]
         return 0
+    # Special Case: simplify(:(x - (-y))) == x + y
+    elseif length(args) == 2 && isminus(args[2])
+        return Expr(:call, :+, args[1], args[2].args[2])
     else
         return Expr(:call, :-, args...)
     end

test/symbolic.jl

@@ -22,7 +22,7 @@
 @test isequal(differentiate(:(exp(x)), :x), :(exp(x)))
 @test isequal(differentiate(:(log(x)), :x), :(1 / x))
 @test isequal(differentiate(:(sin(x) + sin(x)), :x), :(cos(x) + cos(x)))
-@test isequal(differentiate(:(sin(x) - cos(x)), :x), :(-(cos(x),-sin(x)))) # Simplify -(a, -(b)) => +(a, b)
+@test isequal(differentiate(:(sin(x) - cos(x)), :x), :(cos(x) + sin(x)))
 @test isequal(differentiate(:(x * sin(x)), :x), :(sin(x) + x * cos(x)))
 @test isequal(differentiate(:(x / sin(x)), :x), :((sin(x) - x * cos(x)) / (sin(x)^2)))
 @test isequal(differentiate(:(sin(sin(x))), :x), :(*(cos(x),cos(sin(x)))))