[julia-users] I had an idea to (ab)use the type system. It failed miserably. Why?

[andrew cooke]

various people (mainly vtjnash, i guess) have repeatedly warned me off 
using the type system as much as i would like.  despite this, i had the 
idea of sticking iterator parameters into the type system.  my reasoning 
was that specialised code would be inlined for each iterator.  it failed 
miserably (gave a 300x slowdown), but i don't understand why.

so can anyone give me a rough sketch of why the code at 
https://github.com/andrewcooke/IntModN.jl/blob/master/src/Range.jl is such 
a bad idea?

here's the meat:

immutable TypedRange{S,I,E} <: Ranges{Int}
end

# MyInt is used just to isolate this code; if I re-implement the generic
# count I cause chaos, changing code deep within the implementation...

function colon(start::MyInt, inc::MyInt, end_::MyInt)
    TypedRange{start.i, inc.i, end_.i}()
end


start{S,I,E}(r::TypedRange{S,I,E}) = S - I
next{S,I,E}(r::TypedRange{S,I,E}, i::Int) = (i + I, i + I)
done{S,I,E}(r::TypedRange{S,I,E}, i::Int) = I > 0 ? i >= E : i <= E

thanks,
andrew