This assignment involves a little bit of coding with xfst. You'll need to turn in some code and results files, which should be submitted via Canvas.
Using xfst, write a finite-state transducer that can generate and analyze a small set of verbs in all of their inflected forms. This FST will handle two spelling change rules: the rule that deletes a final e before -ing or -ed, and the rule that inserts a k when c appears between a vowel and -ing or -ed. The first rule is provided already. Your job is to write the second.
Note: xfst defines a language for regular expressions which makes it relatively easy to write morphophonological rewrite rules. For this problem, however, you must stay with the basic operators. No credit will be given for answers that use the xfst operator -> or its kin. On the other hand, if you get stuck, you might find it helpful to write the rule in that notation, and then examine the network that xfst produces.
To do this assignment, you'll need the following two files:
Copy them somewhere onto your Patas home directory. Log in to patas and do: 'wget path_to_file'. If you're using Windows to download them, make sure that it doesn't add any new file extensions.
verb_lexicon is the lexicon of verbs (in citation form) that we'll be
working with.
k.xfst is the xfst
script that does the work. It is the file you'll need to modify for this part
of the assignment.
To start xfst, log onto Patas and type "xfst" (your $path variable should already be set appropriately). You'll get an xfst prompt.
To run the script, enter:
source k.xfst
After you've run the script, there should be an FST on the stack. To apply that FST, try:
apply up spruced apply down picnic+ing
Observe that it doesn't yet have the right behavior in the second example.
Modify k.xfst until it has the right behavior. The files produced by
the script (underlying, onerule, tworules and threerules) should be helpful in
testing it as you go. You can also use apply up and apply down
to observe the behavior of the network. Here is a short summary
of xfst syntax.
Hint: If you have trouble, try commenting out the first rule and then writing a most simple rule as a first rule, to make sure you can at least observe some change happening. Make sure you understand the function of the ':' and the concepts fof upper and lower tape.
To examine a network, type:
print net
The network defined in k.xfst is too large to be usefully examined like this, but you might try some others:
read regex [a b c]; print net read regex [a+ b c]; print net read regex [e %+ -> 0 || _ [e|i] ]; print net
Text-to-speech systems rely on large pronunciation dictionaries in combination with rules for unknown words, or for large morphologies. Since English doesn't have a very complex morphology, we will use stress patterns in Spanish to demonstrate the need for pronunciation rules. Spanish has a fairly simple pattern of stress assignment. It is, roughly:
There are complications to this (e.g., some clitics are extra-metrical), but it's not relevant for this assignment. We will focus on stress in verbs. Because Spanish has many verb forms, it can be simpler to use these rules to understand which syllable in the verb contains stress, rather than encoding stress separately for every possible verb form. For the first part of this assignment, we will focus on the present-tense indicative forms for one of the verb declensions, plus the infinitive form. Your job is to write an FST that takes in morpheme-segmented verbs, and outputs a verb form with stress assigned to the correct syllable. We will mark stress with a ' preceding the syllable's vowel(s). The table below gives the verb endings we are considering:
| +o | 1st.SG | +amos | 1st.PL |
| +as | 2nd.SG | +'ais | 2nd.PL |
| +a | 3rd.SG | +an | 3rd.PL |
| +ar | INF |
Note that the 2nd person plural morpheme +'ais comes with stress. This means that the ending "-ais" is always the stressed syllable in the word. Refer to the rules given above to work out the stress pattern for the other forms. For the root habl (speak), the two sides of the FST should look like:
| habl+o | <--> | h'ablo |
| habl+'ais | <--> | habl'ais |
That is, your FST needs to compose the roots given in the root lexicon (see below) with all possible endings, and then run that through a stress-assignment rule(s), to output a string with the stressed syllable marked with a '. Note that this means that (with the exception of the 2nd person plural) stress only occurs on one side of the tape. Also make sure your FST is taking each letter as a separate arc (that is, each arc should be one character, not "'ais" as a single transition!).
[ c a t | d o g ]
Or, using the 'explode' operators, like this:
[ {cat} | {dog} ]
Recall that % is the escape character, and that most punctuation marks have a special meaning in xfst and therefore need to be escaped. A short summary of xfst syntax can be found here.
[ A -> B || C _ D ]
This is the xfst form for A is rewritten as B when it occurs between C and D (where C and D refer to the upper tape context). A, B, C and D are all regular expressions.
Rules that run in parallel are separated by ,, (two commas):
[ A -> B || C _ D ,, E -> F || G _ H ]
Rules of epenthesis (which insert something where nothing was before) are written like this:
[ [..] -> A || B _ C ]This can be read as "nothing goes to A in the context B _ C". If you used 0 (epsilon) instead of [..] in this rule, it would try to insert an infinite number of As between B and C, because there are an infinite number of empty strings between B and C.