Debugging Automa


All Automa's debugging tools are NOT part of the API and are subject to change without warning. You can use them during development, but do NOT rely on their behaviour in your final code.

Automa is a complicated package, and the process of indirectly designing parsers by first designing a machine can be error prone. Therefore, it's crucial to have good debugging tooling.


Revise is not able to update Automa-generated functions. To make your feedback loop faster, you can manually re-run the code that defines the Automa functions - usually this is much faster than modifying the package and reloading it.

Ambiguity check

It is easy to accidentally create a machine where it is undecidable what actions should be taken. For example:

machine = let
    alphabet = re"BC"
    band = onenter!(re"BBA", :cool_band)
    compile(re"XYZ A" * (alphabet | band))

# output
ERROR: Ambiguous NFA.

Consider what the machine should do once it observes the two first bytes AB of the input: Is the B part of alphabet (in which case it should do nothing), or is it part of band (in which case it should do the action :cool_band)? It's impossible to tell.

Automa will not compile this, and will raise the error:

ERROR: Ambiguous NFA.

Note the error shows an example input which will trigger the ambiguity: XYZ A, then B. By simply running the input through in your head, you may discover yourself how the error happens.

In the example above, the error was obvious, but consider this example:

fasta_machine = let
    header = re"[a-z]+"
    seq_line = re"[ACGT]+"
    sequence = seq_line * rep('\n' * seq_line)
    record = onexit!('>' * header * '\n' * sequence, :emit_record)
    compile(rep(record * '\n') * opt(record))

# output
ERROR: Ambiguous NFA.

It's the same problem: After a sequence line you observe \n: Is this the end of the sequence, or just a newline before another sequence line?

To work around it, consider when you know for sure you are out of the sequence: It's not before you see a new >, or end-of-file. In a sense, the trailing \n really IS part of the sequence. So, really, your machine should regex similar to this

fasta_machine = let
    header = re"[a-z]+"
    seq_line = re"[ACGT]+"
    sequence = rep1(seq_line * '\n')
    record = onexit!('>' * header * '\n' * sequence, :emit_record)

    # A special record that can avoid a trailing newline, but ONLY if it's the last record
    record_eof = '>' * header * '\n' * seq_line * rep('\n' * seq_line) * opt('\n')
    compile(rep(record * '\n') * opt(record_eof))
@assert fasta_machine isa Automa.Machine

# output

When all else fails, you can also pass unambiguous=false to the compile function - but beware! Ambiguous machines has undefined behaviour if you get into an ambiguous situation.

Create Machine flowchart

The function machine2dot(::Machine) will return a string with a Graphviz .dot formatted flowchart of the machine. Graphviz can then convert the dot file to an SVG function.

On my computer (with Graphviz and Firefox installed), I can use the following Julia code to display a flowchart of a machine. Note that dot is the command-line name of Graphviz.

function display_machine(m::Machine)
    open("/tmp/", "w") do io
        println(io, Automa.machine2dot(m))
    run(pipeline(`dot -Tsvg /tmp/`, stdout="/tmp/machine.svg"))
    run(`firefox /tmp/machine.svg`)

The following function are Automa internals, but they might help with more advanced debugging:

  • re2nfa - create an NFA from an Automa regex
  • nfa2dot - create a dot-formatted string from an nfa
  • nfa2dfa - create a DFA from an NFA
  • dfa2dot - create a dot-formatted string from a DFA

Running machines in debug mode

The function generate_code takes an argument actions. If this is :debug, then all actions in the given Machine will be replaced by :(push!(logger, action_name)). Hence, given a FASTA machine, you could create a debugger function:

 @eval function debug(data)
    logger = []
    $(generate_code(fasta_machine, :debug))

Then see all the actions executed in order, by doing:

julia> debug(">abc\nTAG")
4-element Vector{Any}:

Note that if your machine relies on its actions to work correctly, for example by actions modifying p, this kind of debugger will not work, as it replaces all actions.

More advanced debuggning

The file test/debug.jl contains extra debugging functionality and may be included. In particular it defines the functions debug_execute and create_debug_function.

The function of create_debug_function(::Machine; ascii=false) is best demonstrated:

machine = let
    letters = onenter!(re"[a-z]+", :enter_letters)
    compile(onexit!(letters * re",[0-9]," * letters, :exiting_regex))
eval(create_debug_function(machine; ascii=true))
(end_state, transitions) = debug_compile("abc,5,d!")
@show end_state

Will create the following output:

end state = -6
7-element Vector{Tuple{Char, Int64, Vector{Symbol}}}:
 ('a', 2, [:enter_letters])
 ('b', 2, [])
 ('c', 2, [])
 (',', 3, [])
 ('5', 4, [])
 (',', 5, [])
 ('d', 6, [:enter_letters])

Where each 3-tuple in the input corresponds to the input byte (displayed as a Char if ascii is set to true), the Automa state reached on reading the letter, and the actions executed.

The debug_execute function works the same as the debug_compile, but does not need to be generated first, and can be run directly on an Automa regex:

julia> debug_execute(re"[A-z]+", "abc1def"; ascii=true)
(-3, Tuple{Union{Nothing, Char}, Int64, Vector{Symbol}}[('a', 2, []), ('b', 3, []), ('c', 3, [])])

Return a String with a flowchart of the machine in Graphviz (dot) format. Using Graphviz, a command-line tool, the dot file can be converted to various picture formats.


open("/tmp/", "w") do io
    println(io, machine2dot(machine))
# Requires graphviz to be installed
run(pipeline(`dot -Tsvg /tmp/`), stdout="/tmp/machine.svg")