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README
lens-th-rewrite
A GHC plugin that rewrites TH splices into regular Haskell functions.
Usage
build-depends: lens-th-rewrite
ghc-options: -fplugin=GHC.Plugin.LensThRewrite
Operation
import Control.Lens
data Person
= Person
{ _name :: String
, _age :: Int
} deriving (Show, Eq)
$(makeLenses ''Person)
becomes
import Control.Lens
data Person
= Person
{ _name :: String
, _age :: Int
} deriving (Show, Eq)
age :: Lens' Person Int
age = lens _age $ \record field -> record { _age = field }
name :: Lens' Person String
name = lens _name $ \record field -> record { _name = field }
Why
Cross-compilation of Haskell code requires TH splices to be executed on the target machine, as opposed to the host. This can be a non-starter for many projects.
Limitations
Only makeLenses is supported. Also, you must define your data type in the same module makeLenses is used.
Preprocessor
It is possible to use the executable bundled with this library as a pre-processor.
Add {-# options_ghc -F -pgmF=lens-th-rewrite-pp #-}
Long term
GHC should split up Template Haskell into pure and impure variants. Allowing pure TH code to be executed on the host, and impure on the target.