On Monday, August 25, 2014 12:28:00 PM UTC-5, John Myles White wrote:
>
> Array{FloatingPoint} isn't related to Array{Float64}. Julia's type system
> always employs invariance for parametric types:
> https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)
>
> <https://www.google.com/url?q=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCovariance_and_contravariance_%28computer_science%29&sa=D&sntz=1&usg=AFQjCNH5Mpuwh71o9dv0_TDx9OcMvvKfWg>
>
To underline this point a bit, it's even a bit worse than that:
Array{FloatingPoint} will work just fine for a lot of things, but it stores
all elements as heap pointers, so array-like operations (such as linear
algebra routines) will often be extremely slow.
As a rule, you almost never use an abstract type as the type parameter of a
parametric type for this reason. Where you wish to be generic over a
specific family of types under an abstract type, you can use type
constraints:
function foo{T<:FloatingPoint}(src::Array{T,1})
...
end
But often type annotations can be omitted completely.
