*No!* This is one of the most common misconceptions about Julia
programming.
The type declarations in function arguments have *no impact* on
performance. Zero. Nada. Zip. You *don't have to declare a type at all*
in the function argument, and it *still* won't matter for performance.
The argument types are just a filter for when the function is applicable.
The first time a function is called, a specialized version is compiled for
the types of the arguments that you pass it. Subsequently, when you call
it with arguments of the same type, the specialized version is called.
Note also that a default argument foo(x, y=false) is exactly equivalent to
defining
foo(x,y) = ...
foo(x) = foo(x, false)
So, if you call foo(x, [1,2,3]), it calls a version of foo(x,y) specialized
for an Array{Int} in the second argument. The existence of a version of
foo specialized for a boolean y is irrelevant.
