Due: Thursday, Sept 29th by 11:59 PM

Getting Started

Download fa.scm. Open it in DrRacket.

Your Task

Your task is to implement the simulation of a deterministic finite automaton, or DFA. The simulation, given a DFA and an input string, will use the DFA to process the string and determine whether the DFA accepts or rejects the string. If the string is accepted, the simulation produces a path (sequence of states) through the DFA, from the start state to a final (accepting) state.

You will need to define three procedures: dfa-is-final-state?, dfa-find-transition, and dfa-simulate.


This program uses the define-datatype construct defined by the eopl variant of Scheme. Read Section 2.4 in the textbook for an overview of how define-datatype works.

Three datatypes are defined: dfa-transition, dfa-path, and dfa.

The dfa-transition datatype represents a single transition in a DFA. A valid DFA transition will specify a from state, a to state, and a symbol. It represents the existence of a transition connecting the from state to the to state, consuming the designated symbol.

The dfa-path datatype represents a sequence of states connected by transitions, starting with the start state. It is represented by a list containing the sequence of states in reverse order.

The dfa datatype represents a DFA. It consists of a designated start state, a designated list of final (accepting) states, and a list of transitions (which are instances of the dfa-transition datatype).

There are detailed comments in fa.scm describing the contents of these data types.

Note that the dfa-transition and dfa-path data types both have an invalid variant.

  • In the case of dfa-transition, the invalid variant is used by the dfa-find-transition procedure to indicate that a transition does not exist.
  • In the case of dfa-path, the invalid variant is used by the dfa-simulate procedure to indicate that an input string is rejected by the DFA


This procedure takes as parameters a dfa and a state, and returns #t if the state is a final state and #f if the state is not a final state.


This procedure takes as parameters a dfa, a from state, and a symbol, and returns

  • a valid transition whose from state and symbol match the ones given as parameters, or
  • an invalid transition if no such transition exists in the DFA


This procedure takes as parameters a dfa and an input string (represented as a list of symbols). It returns

  • a valid path if the string is accepted by the dfa, where the list of states in the path corresponds to the states traversed while processing the string, or
  • an invalid path if the string is rejected by the dfa


Start by implementing the dfa-is-final-state? procedure.

Next, implement the dfa-find-transition procedure.

Finally, implement dfa-simulate. The dfa-is-final-state? and dfa-find-transition procedures should be useful.

You can use the eq? procedure to test two symbol values to see if they are the same symbol:

You can use the = procedure to test whether or not two state values are the same state, since integer values are used to represent states.

Use the dfa-path-extend procedure to append a new state onto an existing path, creating a new (extended) path. This procedure will be useful in your implementation of dfa-simulate.

A global variable called odd-as is an instance of the dfa datatype which recognizes all strings of a's and b's containing an odd number of a's:


A global variable called no-abb is an instance of the dfa datatype which recognizes all strings of a's and b's which do not contain the substring abb:


As you implement each procedure, you can test it using odd-as and no-abb as test DFA instances. Here is a transcript showing the expected results of the procedures (user input in bold):

> (dfa-is-final-state? odd-as 1)
> (dfa-is-final-state? odd-as 2)
> (dfa-is-final-state? no-abb 1)
> (dfa-is-final-state? no-abb 2)
> (dfa-is-final-state? no-abb 3)
> (dfa-find-transition odd-as 1 'a)
#(struct:valid-dfa-transition 1 2 a)
> (dfa-find-transition odd-as 1 'b)
#(struct:valid-dfa-transition 1 1 b)
> (dfa-find-transition no-abb 3 'a)
#(struct:valid-dfa-transition 3 2 a)
> (dfa-find-transition no-abb 3 'b)
> (dfa-simulate no-abb '(b b b b))
#(struct:valid-dfa-path (1 1 1 1 1))
> (dfa-simulate no-abb '(a b a b))
#(struct:valid-dfa-path (3 2 3 2 1))
> (dfa-simulate no-abb '(a a a b))
#(struct:valid-dfa-path (3 2 2 2 1))
> (dfa-simulate no-abb '(a a a b b))


Submit your modified version of fa.scm to Marmoset as assign3:


IMPORTANT: after uploading, you should download a copy of your submission and double-check it to make sure that it contains the correct file(s). You are responsible for making sure your submission is correct. You may receive a grade of 0 for an incorrectly submitted assignment.