visamb — displays the ambiguities in a parse tree represented in AsFix2


visamb [-i file | --input file] [-o file | --output file] [-b] [-S | --silent] [--verbose level] [-k level | --keep level] [-h | -? | --help] [--about] [--version]


The visamb utility extracts ambiguities from a parse tree and displays these in a readable format. Ambiguities are displayed by printing the non-terminals of the nodes of the parse trees in the ambiguities.

The SDF2 implementation caters for arbitrary context-free grammars. That is, even for ambiguous grammars the parser will produce a parse trees containing a concise encoding of all possible parses. Ambiguities are represented by means of amb nodes that contain a list of possible parse trees at that point. For most applications, however, it is desirable to develop unambiguous grammars.


Common Input/Output Options

-i file

The input term given by the file name file.

In the absence of the -i option, input will be read from stdin.

-o file

The output will be written to the file given by the file name file.

In the absence of the -o option, output will be written to stdout.


The output will be written in the binary (BAF) ATerm format.

ATerms in the BAF format require a lot less space than ones in the TAF format, but the Java ATerm library does not currently support baf ATerms. ATerms in the baf format is the preferred format of exchange between Stratego tools.

Common Debugging Options


See --version.

-h, -?, --help

Display usage information.

--keep int

Keep intermediate results produced by the internal stages in the pretty-printing process. This is only useful for debugging. A high value of int indicates increased eagerness for keeping intermediate results.

Default setting is 0, indicating that no intermediates will be kept.

-S, --silent

Silent execution. Same as --verbose 0.

--verbose int

Set verbosity level to numerical value int. The higher the number, the more information about pp-aterm's inner workings are printed.

Alternatively, int can be set to either of the following verbosity levels, given in increasing order of verbosity: emergency, alert, critical, error, warning, notice, info, debug, vomit.


Displays the tool name and version.


Consider the following syntax definition:

  module Main
    sorts Exp
    lexical syntax
      [\ \t\n] -> LAYOUT
    context-free syntax
      "id"    -> Exp
      Exp Exp -> Exp

The following command will convert this into a parse table, suitable for visamb.

$ sdf2table -i Exp.sdf -o Exp.tbl

Next, we can see how the expression id id id is ambiguous with this grammar, by doing:

$ echo "id id id" | sglr -2 -p Exp.tbl | visamb

The result is:

  # ambiguities = 1
  + * id id id





Only the inner ambiguities are displayed, i.e., if a phrase and one of its sub-phrases are ambiguous, only the ambiguities of the sub-phrase are displayed.

Reporting Bugs

Please report bugs to


Copyright (C) 2002-2005 Eelco Visser

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.