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Morphological dictionary

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This page intends to describe how and why the morphological dictionaries in Apertium work the way they do. Morphological dictionaries in Apertium (or more properly lttoolbox) are based on finite-state transducer technology, in this way they can also be referred to as lexical transducers. The task of a morphological dictionary is to model the rules that govern the internal structure of words in a language.

For example, speakers of English realise that the words "dog" and "dogs" are related, that "dogs" is to "dog" as "cats" is to "cat". The rules understood by the speaker reflect specific patterns and regularities in the way in which words are formed from smaller units and how those smaller units interact.

Finite-state transducers are not the only way to model these rules, it is also possible to write the rules in scripting languages such as perl or python, or as a lexer (examples include the Buckwalter morphological analyser for Arabic or IceMorphy for Icelandic). There are however a number of downsides to this method:

  • The analysers created are not reversible, that is, you cannot use the same model to analyse and generate.
  • As the rule content may be both imperative and declarative, programs can be more complicated to understand and edit by non-experts.

In contrast, finite-state transducers are: reversible, the same description can be used for both analysis and generation; declarative, in that a description of the morphological rules is written as separate from the algorithm which processes them. Note that analysers may also be described as decorated tries, or finite-state acceptors (for example hunmorph), this may be declarative, but non-reversible (i.e. not applicable to generation).

A finite-state acceptor for the string "beer".

Finite-state automata

To start with it is worth defining what is a finite-state automaton, and how the two main types differ. This will not be an exhaustive description, just an overview so that the difference can be distinguished for the purposes of this article. To begin with, some terminology, if you are familiar with graphs (as in the data structure) this might help. A finite-state automaton can be visualised as a graph, with the nodes representing states, and the arcs representing transitions.


A finite-state acceptor (or recogniser), as seen to the right, is an automaton which accepts or rejects input strings. So, taking the example at the right, we can see the automaton has:

  • a start state,
  • a number of intermediate states,
  • two final states (denoted by a double border around the node),
  • a number of transitions

We can crudely model this in a programming language such as python in order to get an idea of the behaviour of this automata.

states = ['b', 'e', 'e', 'r']; # Set of states
current_state = 0;             # Start state 
c = sys.stdin.read(1);
while c:                       # Input loop
        if current_state == len(states): # If we've reached the final state
        elif c == states[current_state]: # If the input matches the value of the current state
                current_state = current_state + 1;
        else:                            # If the input does not match the current state and we're not final
        c = sys.stdin.read(1);


  • string
  • alphabet
  • empty string

Specification format




A representation of the transducer for the noun "beer" in English.




See also

Further reading

  • Ortiz-Rojas, S., Forcada, M. L., and Ramírez-Sánchez, G. (2005) "Construcción y minimizacion eficiente de transductores de letras a partir de diccionarios con paradigmas". Procesamiento del Lenguaje Natural, 35, 51–57.
  • A. Garrido-Alenda, M.L. Forcada, (2002) "Comparing nondeterministic and quasideterministic finite-state transducers built from morphological dictionaries", Procesamiento del Lenguaje Natural, (XVIII Congreso de la Sociedad Española de Procesamiento del Lenguaje Natural, Valladolid, Spain, 11-13.09.2002)
  • R.C. Carrasco, M.L. Forcada, (2002) "Incremental construction and maintenance of minimal finite-state automata", Computational Linguistics, 28:2, 207-216
  • Alicia Garrido-Alenda, Mikel L. Forcada, Rafael C. Carrasco, (2002) "Incremental construction and maintenance of morphological analysers based on augmented letter transducers", in Proceedings of TMI 2002 (Theoretical and Methodological Issues in Machine Translation, Keihanna/Kyoto, Japan, March 2002), p. 53-62
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