Deriving Lift

February 10, 2017

There was a proposal two years ago from Ryan Scott for automatic derivation of Lift typeclass. This has been implemented in GHC and is available from 8.0.1.

Writing QuasiQuoter has become much easier and fun now. Let’s try to create UTCTime from QuasiQuoter by parsing a string in YYYY format. That gives the following code:

{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DeriveLift #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Language.Haskell.TH
import Language.Haskell.TH.Quote
import Language.Haskell.TH.Syntax
import Data.Time.Format
import Data.Time

deriving instance Lift Day

instance Lift DiffTime where
  lift x = return $ LitE (IntegerL $ diffTimeToPicoseconds x)

deriving instance Lift UTCTime

time :: QuasiQuoter
time =
  { quoteExp =
    \str ->
       let (item :: Maybe UTCTime) = parseTimeM 
       in [|item|]

Now load the above module in ghci:

λ> :l Time.hs
[1 of 1] Compiling Main             ( Time.hs, interpreted )

λ> :set -XQuasiQuotes
λ> [time|2007|]
Just 2007-01-01 00:00:00 UTC

That was more easier than I thought it to be!