Translating linearization functions to Haskell: support for variants

By adding the flag -haskell=variants to the command line, GF will now generate
linearization functions in Haskell that support variants. Variants are
represented as lists in Haskell.

Variants inside pre { ... } expressions are still ignored.

TODO: apply some monad laws to generate more compact code (using an
intermediate representation of the generated Haskell code, instead of
pretty printing directly from the GF code).

src/runtime/haskell/PGF/Haskell.hs

@@ -2,7 +2,7 @@
 -- with @gf -output-format=haskell -haskell=concrete@
 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-}
 module PGF.Haskell where
-import Control.Applicative((<$>))
+import Control.Applicative((<$>),(<*>))
 import Data.Char(toUpper)
 import Data.List(isPrefixOf)
 import qualified Data.Map as M
@@ -54,9 +54,15 @@ class Has_s r a | r -> a where proj_s :: r -> a
 
 -- | Haskell representation of the GF record type @{s:t}@
 data R_s t = R_s t deriving (Eq,Ord,Show)
-instance (EnumAll t) => EnumAll (R_s t) where
-    enumAll = (R_s <$> enumAll)
+instance (EnumAll t) => EnumAll (R_s t) where enumAll = R_s <$> enumAll
 instance Has_s (R_s t) t where proj_s (R_s t) = t
 
 -- | Coerce from any record type @{...,s:t,...}@ field to the supertype @{s:t}@
 to_R_s r = R_s (proj_s r)
+
+
+-- *** Variants
+
+infixr 5 +++
+
+xs +++ ys = (++) <$> xs <*> ys