> module LSyntax where

> type Var = String  -- only global variables, no tables

> newtype Block =
>     Block [ Statement ]                 -- s1 ... sn 
>   deriving (Eq, Show)

> instance Semigroup Block where
>   (<>) :: Block -> Block -> Block
>   Block s1 <> Block s2 = Block (s1 <> s2)

> instance Monoid Block where
>   mempty :: Block
>   mempty = Block []

> data Statement =
>     Assign Var Expression               -- x = e
>   | If Expression Block Block           -- if e then s1 else s2
>   | While Expression Block              -- while e s 
>   | Try Block Var Block                 -- try s1 handle x in s2 
>   | Throw Expression                    -- throw e
>   deriving (Eq, Show)

> data Expression =
>     Var Var                             -- x
>   | Val Value                           -- v
>   | Op2 Expression Bop Expression       -- e1 op e2
>   deriving (Eq, Show)

> data Bop =
>     Plus     -- `+`  :: Int -> Int -> Int
>   | Minus    -- `-`  :: Int -> Int -> Int
>   | Times    -- `*`  :: Int -> Int -> Int
>   | Divide   -- `//` :: Int -> Int -> Int
>   deriving (Eq, Show, Enum)

> data Value =
>     IntVal Int   -- literal ints
>   | NilVal       -- `nil`
>   deriving (Eq, Show)