No, it can actually offer the guarantees of static typing within statically typed functions. function match_books(bids::Array{Bid,1}, asks::Array{Ask,1})

No, it can actually offer the guarantees of static typing within statically typed functions. function match_books(bids::Array{Bid,1}, asks::Array{Ask,1}) ... end I have static guarantees on the type of bids, asks, and anything else inside `match_books`. Outside, you might attempt to pass nonsense in and get a runtime error. But inside, I can rely on static guarantees and so can the compiler (this is why Julia is fast).