composition of lenses

gistfile1.hs

{-# LANGUAGE TemplateHaskell #-}

import Data.Lens.Template
import Data.Lens.Lazy

data Line = Line {
      _lnBeg :: Point
    , _lnEnd :: Point
    } deriving (Show)

data Point = Point {
      _ptX :: Int
    , _ptY :: Int
    } deriving (Show)
    
$( makeLenses [''Point, ''Line] )

ptX :: Lens Point Int
lnBeg :: Lens Line Point

compose = ptX . lnBeg


test_lens.hs:18:11:
    Couldn't match expected type `b0 -> c0'
                with actual type `Lens Point Int'
    In the first argument of `(.)', namely `ptX'
    In the expression: ptX . lnBeg
    In an equation for `compose': compose = ptX . lnBeg
Failed, modules loaded: none.

Note: