I'm using julia 0.4.1. Here's an example
using ForwardDiff
foo(l, x) = ifelse( abs(x) < l, sin(x), cos(x) )
bar(l, x) = ifelse( isless(abs(x),l), sin(x), cos(x) )
derivative(x->foo(2,x), 1.) # ERROR: < not defined for ForwardDiff.
GradientNumber{1,Float64,Tuple{Float64}}
derivative(x->bar(2,x), 1.) # 0.5403023058681398
derivative(x->foo(2,x), 3.) # ERROR: < not defined for ForwardDiff.
GradientNumber{1,Float64,Tuple{Float64}}
derivative(x->bar(2,x), 3.) # -0.1411200080598672
immutable FooType
v
end
value(f::FooType) = f.v
Base.isless(f::FooType, x::Real) = value(f) < x
Base.isless(x::Real, f::FooType) = x < value(f)
FooType(3) < 4 # true
FooType(3) < 1 # false
1 < FooType(3) # true
3 < FooType(3) # false
