Add initial HW code for HW 5.

KarelSyntax.hs

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+-- | This module defines the syntax of the Karel language.
+module KarelSyntax where
+
+
+-- | A Karel program is a list of macro definitions and a statement to
+--   use as the body of the "main" function.
+type Prog = (Defs,Stmt)
+
+-- | A macro name.
+type Macro = String
+
+-- | A list of macro definitions.
+type Defs = [(Macro,Stmt)]
+
+-- | Cardinal directions.
+data Card = North | South | East  | West  deriving (Eq,Show)
+
+-- | Directions relative to the current facing.
+data Dir  = Front | Back  | Right | Left  deriving (Eq,Show)
+
+-- | Environment queries.
+data Test = Not    Test   -- boolean negation
+          | Facing Card   -- am I facing the given cardinal direction?
+          | Clear  Dir    -- can I move in the given relative direction?
+          | Beeper        -- is there a beeper here?
+          | Empty         -- is my beeper bag empty?
+  deriving (Eq,Show)
+
+-- | Statements.
+data Stmt = Shutdown                 -- end the program
+          | Move                     -- move forward
+          | PickBeeper               -- take a beeper
+          | PutBeeper                -- leave a beeper
+          | Turn    Dir              -- rotate in place
+          | Call    Macro            -- invoke a macro
+          | Iterate Int  Stmt        -- fixed repetition loop
+          | If      Test Stmt Stmt   -- conditional branch
+          | While   Test Stmt        -- conditional loop
+          | Block   [Stmt]           -- statement block
+  deriving (Eq,Show)