Expression.hs

{-# LANGUAGE CPP #-}

-- | Interface to pure expressions

module Language.Embedded.Expression where



import Data.Typeable

import GHC.Prim (Constraint)

import Language.C.Quote.C (ToIdent (..))



-- | Variable identifier
type VarId = String

-- | Expressions that support injection of values and named variables
class FreeExp exp
  where
    -- | Constraint on the types of values and variables in an expression
    -- language
    type VarPred exp :: * -> Constraint

    -- | Construct a value expression
    valExp :: VarPred exp a => a -> exp a

    -- | Construct a named variable expression
    varExp :: VarPred exp a => VarId -> exp a

-- | Value
data Val a
    = ValComp VarId  -- ^ Symbolic value
    | ValEval a      -- ^ Concrete value
  deriving Typeable

instance ToIdent (Val a) where toIdent (ValComp r) = toIdent r

-- | Convert a value to an expression
valToExp :: (VarPred exp a, FreeExp exp) => Val a -> exp a
valToExp (ValComp v) = varExp v
valToExp (ValEval a) = valExp a

-- | Expressions that support evaluation
class FreeExp exp => EvalExp exp
    -- The super class is motivated by the fact that evaluation of functions
    -- `exp a -> exp b` can be done by constructing an argument using `valExp`.
  where
    -- | Evaluation of a closed expression
    evalExp :: exp a -> a