{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}
-- | Symantic for @If@.
module Language.Symantic.Lib.If where

import qualified Data.Text as Text

import Language.Symantic
import Language.Symantic.Lib.Bool (tyBool)
import Language.Symantic.Lib.Function (a0)

-- * Type 'If'
data If

-- * Class 'Sym_If'
type instance Sym If = Sym_If
class Sym_If term where
	if_ :: term Bool -> term a -> term a -> term a
	default if_ :: Sym_If (UnT term) => Trans term => term Bool -> term a -> term a -> term a
	if_ = trans3 if_

-- Interpreting
instance Sym_If Eval where
	if_ (Eval b) ok ko = if b then ok else ko
instance Sym_If View where
	if_ (View cond) (View ok) (View ko) =
		View $ \po v ->
			parenInfix po op $
			Text.concat
			 [ "if ", cond (op, SideL) v
			 , " then ", ok (op, SideL) v
			 , " else ", ko (op, SideL) v ]
		where op = infixN 2
instance (Sym_If r1, Sym_If r2) => Sym_If (Dup r1 r2) where
	if_  = dup3 @Sym_If if_

-- Transforming
instance (Sym_If term, Sym_Lambda term) => Sym_If (BetaT term)

-- Typing
instance ClassInstancesFor If
instance TypeInstancesFor If

-- Compiling
instance Gram_Term_AtomsFor src ss g If
 -- TODO: some support for if-then-else or ternary (?:) operator
instance ModuleFor src ss If

-- ** 'Type's

-- ** 'Term's
teIf_if :: TermDef If '[Proxy a] (() #> (Bool -> a -> a -> a))
teIf_if = Term noConstraint (tyBool ~> a0 ~> a0 ~> a0) $ teSym @If $ lam3 if_