LitExpr = Class {.i = Int}
LitExpr.
    new i = Self::__new__ {.i;}
LitExpr|<: Show|.
    show self = "{self.i}"
AddExpr = Class {.lhs = Expr, .rhs = Expr}
AddExpr.
    new lhs, rhs = Self::__new__ {.lhs; .rhs}
AddExpr|<: Show|.
    show self = "{self.lhs} + {self.rhs}"
SubExpr = Class {.lhs = Expr, .rhs = Expr}
SubExpr.
    new lhs, rhs = Self::__new__ {.lhs; .rhs}
SubExpr|<: Show|.
    show self = "{self.lhs} - {self.rhs}"
PosExpr = Class {.expr = Expr}
PosExpr.
    new expr = Self::__new__ {.expr;}
PosExpr|<: Show|.
    show self = "+{self.expr}"
NegExpr = Class {.expr = Expr}
NegExpr.
    new expr = Self::__new__ {.expr;}
NegExpr|<: Show|.
    show self = "-{self.expr}"

Expr = Enum LitExpr:
    AddExpr
    SubExpr
    NegExpr
Expr.
    lit = LitExpr.new |> Self.new
    add = AddExpr.new |> Self.new
    eval self =
        match self:
            (l: Expr.LitExpr) -> l.i
            (a: Expr.AddExpr) -> a.lhs + a.rhs
            (s: Expr.SubExpr) -> s.lhs - s.rhs
            (p: Expr.PosExpr) -> p.expr
            (n: Expr.NegExpr) -> -n.expr

expr = Expr.add Expr.lit(1), Expr.lit(2)
print! expr # 1 + 2
assert expr.eval() == 3