4.5 Pretty-printing

For pretty-printing, KHEPERA uses a modification of the algorithm presented by [9]. This algorithm is linear in space and time, and does not backtrack when printing. The implementation was straightforward, with modifications added to support source line tracking and formatted pretty-printing. Other algorithms for pretty printing, some of which support a finer-grain control over the formatting, are presented in [7,2,15,16].

For each node type in the AST, a short description, using printf-like syntax, tells how to print that node and its children. If the node can have several different numbers of children, several descriptions may be present, one for each variation. List nodes may have an unknown number of children. Multiple descriptions may be present for multiple languages, with ``fallback'' from one language to another specified at printing time (so, Fortran may be printed for all of those nodes that have Fortran-specific descriptions, with initial fallback to unlabeled nodes (perhaps for C or for the original DSL), and with final fallback to generic S-expressions). This fallback scheme provides usable pretty-printing during development, even before the complete pretty-printer description is finished and debugged.

For printing which requires local analysis, implementor-defined functions can be used to return pre-formatted information or to force a line break. These functions are passed a pointer to the current node, so they have access to the complete AST from the locus being printed. While the pretty-printer is source-language independent and is unaware of the specific application-defined attributes present on the AST, the implementor-defined functions have access to all of this information. We typically use these functions to format type information or to add comments to the generated source codes.

Additional pretty-printer description syntax allows line breaks to be declared as ``inconsistent'' or ``consistent''; allows for forced line breaks; and permits indentation adjustment after breaks.