Pretty Printer

F# features some tools for pretty printing (any kind of data, including trees). This article explains how to use StructuredFormat library and provides a complete example.


F# features some tools for pretty printing. StructuredFormat is a customizable layout engine. Types that wish to customize their structured format specification should implement the IFormattable interface. This interface must provide a method (GetLayout) that outputs a Layout value. StructuredFormat library has several functions and operators to simplify output.

To print a value using this engine, you can use one of the following functions:

  • print_any ('a -> unit)
  • prerr_any ('a -> unit)
  • output_any (out_channel -> 'a -> unit)
  • any_to_string ('a -> string)
  • printf functions with %A format

StructuredFormat is also the engine used by the interactive mode. A layout describes how tokens will fit next to each other and where the breaks, spaces and indentations will be (or can be). Objects such as numbers and strings can be left unformatted, which lets the engine control how they will be printed (for example, culture specific format). The engine can also be customized: print width, depth, culture and floating-point number formatting...


You should have a look in the sformat documentation and see the list of available functions. To convert an atom to a Layout, we might use following functions:

  • objL is layout for most values (int, float according to culture...). Since this function requires an object, you might need to box the value first.
  • wordL, sepL, leftL and rightL convert a string into a Layout. wordL is a string leaf, that will be separated by spaces. leftL and rightL are used when the string behaves like a left (or right) parenthesis: there's a space only on the right (or the left). sepL is used for separators: no spaces is needed.
  • listL, spaceListL, commaListL... are nice functions for printing lists.

Several operators are available to group different layouts. You can choose if layouts are unbreakable (no carriage return), breakable (the engine chooses what's the best) or broken (to start on a new line). You can also choose an indentation, when return line is possible.

  • $$ unbreakable
  • ++ breakable (no indent)
  • -- breakable (indent=1)
  • --- breakable (indent=2)
  • @@ broken (no ident)
  • @@- broken (ident=1)
  • @@-- broken (ident=2)

Pay attention to operators priority. For example, ++ has a higher priority than $$, this may be confusing. You may add extra-parentheses to make things clearer. Here is a function example:

let get_layout (env: #IEnvironment) (e: #IFormattable) = e.GetLayout(env)

type ast =
 | Val of int
 | Var of string
 | BinOp of binop * ast * ast
 | UnOp of unop * ast
   interface StructuredFormat.IFormattable with
      member x.GetLayout(env) =
         match x with
          | Val i -> objL (box i)
          | Var s -> wordL s
          | BinOp (Custom (s, _), t1, t2) ->
               wordL s ++ get_layout env t1 ++ get_layout env t2
          | BinOp (b, t1, t2) ->
               (leftL "(" $$ get_layout env t1) ++ get_layout env b ++
                 (get_layout env t2 $$ rightL ")")
          | UnOp (b, t1) ->
               get_layout env b $$ get_layout env t1

The get_layout function defined above can be removed, but you might need to add type annotations in the code. If you want to test how your printer works, I suggest you to try in the interactive mode. You can set the value fsi.PrintWidth to see how indentation behaves.


Here are two examples to explain how to format a "if" expression in a language.

Source code
 | If (cond, exp1, exp2) ->
      ((wordL "if" $$ get_layout env cond $$ wordL "then")
       @@- (get_layout env exp1))
       @@ wordL "else"
       @@- (get_layout env exp2)
if exp then
Source code
 | If (cond, exp1, exp2) ->
      (wordL "if" $$ get_layout env cond) --
         (wordL "then" -- (get_layout env exp1))
         (wordL "else" -- (get_layout env exp2))
if exp then 1
       else 2

Full example

I've written a full example using a pretty printer. The example also features operators overloading, operators redefinition and AST manipulations. The code contains a tree describing arithmetic expressions (with variables). The AST can be printed and simplified using arithmetic rules (x * 0 = 0, etc.).