ghci> distance (x, x) $ toVector (y, y)
12.727922061357855

I don't understand why this doesn't work:

ghci> let z = toVector (y, y)
interactive>:32:9:
    No instance for (Vectorizable (Double, Double) v0)
      arising from a use of `toVector'
    The type variable `v0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there is a potential instance available:
      instance Vectorizable (Double, Double) (Double, Double)
        -- Defined at vector.hs:13:10
    Possible fix:
      add an instance declaration for (Vectorizable (Double, Double) v0)
    In the expression: toVector (y, y)
    In an equation for `z': z = toVector (y, y)

It seems odd that "toVector" works when used as an argument to
"distance" but not when used in a let expression.

Can anyone explain?