import edu.uwm.cs.gll.RegexParsers

object Fun1Parser extends RegexParsers {

// %%

lazy val program = seq

lazy val seq = (expr + “;”) ^^ Sequence

lazy val expr: Parser[Expression] = (
expr ~ “+” ~ term ^^ { (e1, _, e2) => Add(e1, e2) }
| expr ~ “-” ~ term ^^ { (e1, _, e2) => Subtract(e1, e2) }
)

lazy val term: Parser[Expression] = (
term ~ “*” ~ factor ^^ { (e1, _, e2) => Mult(e1, e2) }
| term ~ “/” ~ factor ^^ { (e1, _, e2) => Div(e1, e2) }
)

lazy val factor: Parser[Expression] =
factor ~ “=” ~ exprBase ^^ { (e1, _, e2) => Equal(e1, e2) }

lazy val exprBase = (
“\\” ~> (id + “,”) ~ “->” ~ expr ^^ { (p, _, e) => Lambda(p, e) }
| “let” ~> id ~ “=” ~ expr ^^ { (id, _, e) => Let(id, e) }
| id ~ (”(” ~> (expr + “,”) expr ~ “then” ~ expr ~ “else” ~ expr ^^ { (e1, _, e2, _, e3) => If(e1, e2, e3) }

| id ^^ Var
| strLit ^^ Lit
| intLit ^^ Lit
)

val id = “”"[a-zA-Z_][a-ZA-Z0-9_]*”"”r

val strLit = “”"”([^\\"]|\\.)*”"”"r

val intLit = “”"\d+”"”.r ^^ { _.toInt }

// %%
}

For the record, I had to make-up/assume a lot of things in order to create this. The definition of an identifier, operator precedence, string semantics, etc. A rough E/BNF grammar would help.