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factorizing Type1_From ast Type0
[haskell/symantic.git] / Language / Symantic / Expr / Alt.hs
1 {-# LANGUAGE ConstraintKinds #-}
2 {-# LANGUAGE DataKinds #-}
3 {-# LANGUAGE FlexibleContexts #-}
4 {-# LANGUAGE FlexibleInstances #-}
5 {-# LANGUAGE GADTs #-}
6 {-# LANGUAGE KindSignatures #-}
7 {-# LANGUAGE MultiParamTypeClasses #-}
8 {-# LANGUAGE Rank2Types #-}
9 {-# LANGUAGE ScopedTypeVariables #-}
10 {-# LANGUAGE TypeFamilies #-}
11 {-# LANGUAGE TypeOperators #-}
12 {-# LANGUAGE UndecidableInstances #-}
13 module Language.Symantic.Expr.Alt where
14
15 import Language.Symantic.Type
16 import Language.Symantic.Expr.Root
17
18 -- * Type 'Expr_Alt'
19 -- | Expression making an alternative between two expressions.
20 data Expr_Alt curr next (root:: *)
21 = Expr_Alt_Curr (curr root)
22 | Expr_Alt_Next (next root)
23
24 -- | Convenient alias. Requires @TypeOperators@.
25 --
26 -- TODO: see if using a type-level list is better.
27 type (.|.) = Expr_Alt
28 infixr 5 .|.
29 type instance Root_of_Expr (Expr_Alt curr next root) = root
30 type instance Type_of_Expr (Expr_Alt curr next root)
31 = Type_of_Expr_Alt (Type_of_Expr (curr root))
32 (Type_of_Expr (next root))
33
34 -- ** Type family 'Type_of_Expr_Alt'
35 -- | Remove 'No_Type' type when building 'Type_of_Expr'.
36 type family Type_of_Expr_Alt
37 (type_curr:: (* -> *) -> * -> *)
38 (type_next:: (* -> *) -> * -> *)
39 where
40 Type_of_Expr_Alt No_Type next = next
41 Type_of_Expr_Alt curr No_Type = curr
42 Type_of_Expr_Alt curr next = Type_Alt curr next
43
44 -- ** Type family 'Is_Last_Expr'
45 -- | Return whether a given expression is the last one in a given expression stack.
46 --
47 -- NOTE: each expression parser uses this type family
48 -- when it encounters unsupported syntax:
49 -- to know if it is the last expression parser component that will be tried
50 -- (and thus return 'Error_Expr_Unsupported')
51 -- or if some other expression parser component shall be tried
52 -- (and thus return 'Error_Expr_Unsupported_here',
53 -- which is then handled accordingly by the 'Expr_From' instance of 'Expr_Alt').
54 type family Is_Last_Expr (ex:: *) (exs:: *) :: Bool where
55 Is_Last_Expr ex ex = 'True
56 Is_Last_Expr ex (Expr_Root exs) = Is_Last_Expr ex (exs (Expr_Root exs))
57 Is_Last_Expr (ex root) (Expr_Alt ex next root) = 'False
58 Is_Last_Expr other (Expr_Alt curr next root) = Is_Last_Expr other (next root)