向量空间库和约束标量类型

Question

I'm working on a program that uses the vector-space library, and I'm having some troubles with using it. See the code below.

import Data.VectorSpace
-- scale a vector with a float
step :: (VectorSpace a) => a -> Float -> a
step x dt = x ^* dt

When compiling this code segment I get errors regarding the associated scalar type for the vector typeclass.

Could not deduce (Scalar a ~ Float)
from the context (VectorSpace a)
  bound by the type signature for
        step :: VectorSpace a => a -> Float -> a
  at Test.hs:5:9-42 
In the expression: x ^* dt
In an equation for `step': step x dt = x ^* dt

Is there a type signature that will fix this compiler error? Or is there a better library to use for descibing the operations that I'm looking for in a type (like addition and scaling)? In the end I'm hoping to use the code like for things.

step (1,1) 0.5
step 1 0.5

Essentially I'm hoping to reuse some of the instances that vector-space defines.

Answer 1

EDIT: found signature on hackage to be incorrect

You can just add the constraint about which GHC complained:

{-# LANGUAGE GADTs #-}
import Data.VectorSpace

step :: (VectorSpace a, Scalar a ~ Float) => a -> Float -> a
step x dt = x ^* dt