Pretty-printing is not trivial.

Pretty-printing a piece of code probably can be defined as follows:

  1. Line width defined as finite \(N\)
  2. Unless some item is mandatory to begin at new line, a segment of code should be in one line if it will fit
  3. If a segment of code cannot fit in one line, it should be broken down into smaller fragments and aligned at left side appropriately

To realise (3), it is important to identify fragments of equal level, or equivalently, group logically contiguous tokens into same fragment.

Oppen (1979) devised the following algorithm:

  1. scan function to produce (1) token and its length, or (2) the beginning of a group of tokens and the total length, including the spaces added between tokens within, or (3) a space between token the size of the space plus length of next token. Scan function need to lookahead
  2. a buffer stream (FIFO) to store tokens seen. when scan function computed the length \(\ell\) for token \(x\) at the left end of the buffer, it prints and removes \(x\)
  3. print function decides how to print a token using the length information: (1) string is printed immediately, or (2) beginning of a group prints nothing but pushes the current indentation on a stack, or (3) end of a group pops the indentation from a stack, or (4) for a blank, check to see if the next block can fit in the current line, print a blank if so or skip to next line with indentation specified by the top-of-stack plus arbitrary offset.

In developing pretty-printers, it is preferred to avoid back-tracking for performance reasons.

Jackson et al (2008) outlines a structure for pretty printing: The abstract syntax tree (AST) produced by parser should be converted into a layout options tree (LOT) by layout options generator (LOG).


  1. Bob Boyer (1973) Pretty-print, Memo No 64
  2. Derek C. Oppen (1979) Pretty printing. Stanford University, Stanford Verification Group Report No 13 / Computer Science Department Report No STAN-CS-79-770
  3. Olaf Chitil (2012) Efficient simple pretty printing combinators
  4. Jackson et al (2008) Stable, flexible, peephole pretty-printing. Elsevier Science
  5. Swierstra and Chitil (2009) Linear, Bounded, Functional Pretty-Printing. J Functional Programming, 19(01):182-196, Jan