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

Expr = Enum:
    LitExpr
    AddExpr
    SubExpr
    NegExpr
Expr.
    lit = Self.cons(LitExpr)
    add = Self.cons(AddExpr)
    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