Fast, type-safe vector libary for Purescript inspired by Idris. A vector is list with its size encoded in the type.
spago install fast-vect
A vector is a list (or an Array in the case of Purescript) that has its size encoded in the type. For instance in Idris you can define a vector like this:
vect : Vect 3 Int vect = [1,2,3]
Note the value
3 in the type position, indicating that the vector has exactly three elements.
purescript-fast-vect we can define the same three element vector like this:
vect :: Vect 3 Int vect = 1 : 2 : 3 : empty
What is this good for you ask? Well, now that we have the size encoded in the type, the compiler can help us find errors. For instance, the compiler can tell us when we try to access the head of an empty list.
This leads to a slightly different design of the api. E.g. the
head function in
Data.Array has the following type signature:
head :: forall a. Array a -> Maybe a
So if you call head on an
Array, you have to handle the
Data.FastVect has the following type signature (conceptually - the real one is slightly more complex) :
head :: forall m elem. Vect m elem -> elem
You will get an
elem back, no need to handle a
Maybe. And this operation is always safe, because in the case that the vector is empty you will get a compile-time error.
index function has the following type signature (conceptually - the real one is slightly more complex):
index :: forall i m elem. Term i -> Vect m elem -> elem
If the index
i (represented as a typelevel symbol) is in bounds, i.e.
i < m, you will get an
elem back, otherwise you will get a compile-time error.
Further functions that are defined differently to the
Array functions are:
takeis guaranteed to return you a vector with the number of elements requested and result in a compile-time error if you are trying to request more elements than are in the vector.
dropis guaranteed to drop the exact number of elements from the vector and result in a compile-time error if you are trying to drop more elements than exist in the vector.
You can find the full api on pursuit.
import Prelude import Data.FastVect.FastVect (Vect) import Data.FastVect.FastVect as FV import Typelevel.Arithmetic.Add (Term, term) as :: Vect 300 String as = FV.replicate (term :: Term 300) "a" -- Note you could also leave out the Term type annotation, as PS can infer it: -- as = FV.replicate (term :: _ 300) "a" bs :: Vect 200 String bs = FV.replicate (term :: Term 200) "b" cs :: Vect 500 String cs = FV.append as bs ds :: Vect 2 String ds = cs # FV.drop (term :: Term 299) # FV.take (term :: Term 2) x :: String x = FV.index (term :: Term 499) cs y :: String y = FV.head (FV.singleton "a") big1 :: Vect 23923498230498230420 String big1 = FV.replicate (term :: Term 23923498230498230420) "a" big2 :: Vect 203948023984590684596840586 String big2 = FV.replicate (term :: Term 203948023984590684596840586) "b" big :: Vect 203948047908088915095071006 String big = FV.append big1 big2 -- Note the big example will blow up during runtime.
In order of contribution: