Union/Sum types
===============

How about allowing multiple `data` blocks with names:

For example, in file `Expr.slul`:

    # Common data
    data
        int line
        int column
    end

    data Unary
        Expr op
    end

    data Binary
        Expr op1
        Expr op2
    end

    data IntegerLiteral
        bool negative
        uint64 value
    end

    data StringLiteral
        # some kind of duplicate handling is necessary.
        # maybe there should be a single identifier tree,
        # and then a subtree if it's a union field.
        # perhaps use (fieldname,union-index) as the key in the ident tree?
        String value
    end

    func print
        PrintWriter pw
        int indent
    # is this necessary? can't we just assume that all non-functions are
    # instance variables?
    #   * in that case, maybe typeidents would need to be prefixed?
    #   * or, just require that fields are declared before functions
    accesses
        Unary.operand
        Binary.operand1
        Binary.operand2
        IntegerLiteral.negative
        IntegerLiteral.value
        StringLiteral.value
    code
        print_indent pw indent
        switch this.kind
        case Unary
            pw.print "unary("
            op.print pw (indent+1)
        case Binary
            pw.print "binary("
            op1.print pw (indent+1)
            op2.print pw (indent+1)
        case IntegerLiteral
            pw.print "int_literal("
            pw.print (switch negative [true "-"] [false ""]) number.to_str
        case StringLiteral
            pw.print "string_literal("
            pw.string "\"" value.c_string_escape() "\""
        end
        pw.println ")"
        # it could also be (would look nicer in this case, but if only
        # printing from one function then it will not align as nicely)
        ")".println pw
        # or, have some kind of special coroutine that yields stuff to print?
        yield "unary("
        yield (op.to_str (indent+1))
        yield .newline
        # alternatively, one could simply use different names:
        pw.print "unary("
        op.dump pw (indent+1)
        # or
        pw.print "unary("
        op.print_to pw (indent+1)
    end


More compact syntax:

    data
        int line
        int column
    case Unary
        Expr op
    case Binary
        Expr op1
        Expr op2
    case IntegerLiteral
        bool negative
        uint64 value
    case StringLiteral
        # some kind of duplicate handling is necessary.
        # maybe there should be a single identifier tree,
        # and then a subtree if it's a union field.
        # perhaps use (fieldname,union-index) as the key in the ident tree?
        String value
    end

    func dump
        io PrintWriter pw
        int indent
    code
        dump_indent pw indent
        specific
        pw.println
    case Unary
        pw.print "unary("
        op.print pw (indent+1)
    case Binary
        pw.print "binary("
        op1.print pw (indent+1)
        op2.print pw (indent+1)
    case IntegerLiteral
        pw.print "int_literal("
        pw.print (switch negative [true "-"] [false ""]) number.to_str
    case StringLiteral
        pw.print "string_literal("
        pw.string "\"" value.c_string_escape() "\""
    end

Keyword
-------

* case
* only
* subtype
* when