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.

In 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 Maybe. In contrast, head in 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.

Similarly, the 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.

Furhter functions that are defined differently to the Array functions are:

• take is 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.
• drop is 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.