为GHC中的O（1）预分配Data.Vector？

Question

In C++ world there is an optimisation for std::vector when push_back 'ing known amount of items to new vector - preallocate space

Is there analogue for Haskell unboxed vectors? Doc says cons is O(n) .

Answer 1

Data.Vector.(++) is O(m+n) according to the docs, which is more efficient than cons -ing n times. So if you just put the new elements in a new vector, you can append that to the old one efficiently.

Note that in C++ you might do this in only O(n) provided you have enough preallocated area. The extra O(m) you get in Haskell, I guess, is due to Data.Vector being immutable, and requiring a copy.

If you want to use mutable vectors instead, have a look to Data.Vector.Mutable.grow .