| Abstract: | Unparsing is the problem of transforming an internal representation of a program into an external, concrete syntax. In conjunction with prettyprinting, it is useful for generating readable programs from internal representations. If the target language uses prefix and postfix operators, the problem is nontrivial. This paper shows how to unparse expressions using a simple, bottom-up tree walk, which keeps track of the least tightly binding operator not enclosed by parentheses. During the tree walk, this operator is compared with the operator of the parent expression, and parentheses are inserted based on the precedence, associativity, and fixity (infix, prefix, or postfix) of the two operators. The paper is a literate program. It includes code for the unparser and for its inverse parser, both of which can handle operators of dynamically chosen precedence and associativity. Supporting such operators is useful for languages like ML, in which programmers may assign precedence and associativity to their own functions. © 1998 John Wiley & Sons, Ltd. |
