I am trying to use Data.Vector.Fixed as the input to a function and am struggling to specify the vector's dimension in the type signature.
For example, I am trying to do something like the following:
acronym :: Vector (Peano 8) String -> String
acronym = foldl1 (++)
and thus specifying the vector's dimension (8) using peano numbers as defined in Data.Vector.Fixed.Cont.
However, attempting to compile this gives kind-mismatch errors:
...:12: error:
* Expected a type, but
`Vector (Peano 8) String' has kind
`Constraint'
* In the type signature:
acronym :: Vector (Peano 8) String -> String
|
61 | acronym :: Vector (Peano 8) String -> String
| ^^^^^^^^^^^^^^^^^^^^^^^
...:20: error:
* Expected kind `* -> *', but `Peano 8' has kind `PeanoNum'
* In the first argument of `Vector', namely `(Peano 8)'
In the type signature: acronym :: Vector (Peano 8) String -> String
|
61 | acronym :: Vector (Peano 8) String -> String
How can I specify a fixed vector's size in a type signature?
You're probably looking for something like:
{-# LANGUAGE DataKinds, FlexibleContexts, GADTs #-}
import qualified Data.Vector.Fixed as V
acronym :: (V.Vector v String, V.Dim v ~ 8) => v String -> String
acronym = V.foldl1 (++)
The type class constraint Vector v String indicates that v is a vector with elements of type String, while the constraint Dim v ~ 8 ensures it has the right size.
It can be used with specific vector types, like boxed or continuation vectors, like so:
import qualified Data.Vector.Fixed.Boxed as BV
import qualified Data.Vector.Fixed.Cont as CV
eight = ["one","two","three","four","five","six","seven","eight"]
main = do
print $ acronym (V.fromList eight :: BV.Vec 8 String)
print $ acronym (V.fromList eight :: CV.ContVec 8 String)