%name calc              -- name of generated parsing function
%tokentype { Token }    -- type of accepted tokens
                        -- calc :: [Token] -> t with t type of attribute of
                        -- first rule

%token '+' { PLUS }
       '*' { MULT }
       '(' { LPAREN }
       ')' { RPAREN }
       id  { ID }
       num { NUM }

%%                      -- as in yacc

program : exp               {}

exp : exp '+' term          {}
    | term                  {}

term : term '*' factor      {}
     | factor               {}

factor : id                 {}
       | num                {}
       | '(' exp ')'        {}

{
happyError :: [Token] -> a
happyError ts = error $ "Parse error: " ++ show ts

data Token = ID | NUM | PLUS | MULT | LPAREN | RPAREN | EOF
 deriving Show
}

-----------------------------------------------------------------------------
-- Generate parser with Happy by
--
--     > happy exp.y
--
-- Use the parser by loading the generated program `exp.hs` and
-- evaluating, e.g.,
--
--     ghci> calc [ID,MULT,LPAREN,ID,PLUS,NUM,RPAREN]
--
-----------------------------------------------------------------------------