How do I make lenses from a record in GHCi

Question

I want to play around with the Lens library a bit. I\'ve loaded it into GHCi and created a record data type with the appropriate underscores:

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Answer 1

Tested in GHCi 7.8.3:

:set -XTemplateHaskell
:m +Control.Lens
:{
data AST = AInt  { _aid :: Int, _ival :: Int }
         | AChar { _aid :: Int, _cval :: Char }
         deriving (Show)
makeLenses ''AST
:}

(I believe that the :{ ... :} block is necessary for makeLenses to work).

Let's briefly check:

λ >> AChar 100 'f' ^. aid
100
λ >> AChar 100 'f' ^? cval
Just 'f'
λ >> AInt 101 0 ^? cval
Nothing