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fst-compiler - Two compilers for SFST programs

fst-compilergrammar-file[output-file]fst-compiler-utf8grammar-file[output-file]

-cStore the transducer in compact format which is used by fst- infl2 and.-lStore the transducer in lowmem format.-sSwitch upper and lower level of the transducer. You have to use this switch in order to usefst-infl(fst-infl2,fst-infl3)for generation rather than analysis.

fst-compileris a compiler for finite-state transducer programs. It generates a minimized finite state transducer which can be used withfst-mor,fst-infl,fst-print,fst-compare,fst-parse,andfst-lattice.The compact transducer representation which is generated with the -c flag, is supported byfst-infl2,fst-train,andfst-match.The memory- efficient transducer representation which is generated with the -l flag, is only supported byfst-infl3.The first program argument is the name of a file which contains the transducer program. The programming language is described below. The second argument is the name of the file to which the resulting transducer will be written in binary form. If a second argument is missing, the output will be written tostdout.fst-compiler-utf8differs fromfst-compileronly in the character encoding.fst-compiler-utf8supports UTF8 encoding of the source files whereasfst-compileris to be used for 8-Bit character codes like latin1 which are an extension of the ASCII code. Information about the encoding is stored in the transducer files and used by the other SFST programs.

A transducer program consists of an (optional) sequence ofalphabetandvariabledefinitions followed by a singletransducerexpressionwhich defines the result transducer. Alphabet An alphabet definition consists of the keyword ALPHABET followed by = and some transducer expression e.g. ALPHABET = [a-z]:[A-Z] This command redefines the alphabet as the set of symbol pairs occurring on the transitions of the transducer. Occurrences of two- level operators, negation operators and unquoted periods always have to be preceded by an alphabet definition. Variables There are two different types of variables.Symbolsetvariablesare enclosed by hash signs (#) and take symbol sequences (see below) as values: #UC# = A-Z #LC# = a-zTransducervariablesare enclosed by dollar signs and take transducer expressions as values: $MAP$ = [a-z]:[A-Z]+ $MAP$ = [#LC#]:[#UC#]+ Variables whose name starts with the symbol ‘=’ are specialagreementvariables. If an agreement variable occurs more than once in a transducer expression, it will always have the same value. Consider the following transducer program: $=1$ = [abc] $=1$ X $=1$ The result transducer recognizes the strings aXa, bXb, and cXc. Only acyclic transducers (i.e. transducers with a finite set of string mappings) can be assigned to agreement variables. Symbols A symbol is either - a single character like A s 5, - a quoted character like \* or \_, - a multi-character symbol like <X> or <ab.c5> (which is always enclosed in angle brackets) or - a backslash followed by a number which is the numeric code of the designated character - the null symbol <>. Symbol sequence A symbol sequence is a sequence of characters, multi-character symbols and character ranges, e.g. a-z \. <x>. symbol range A symbol range is either - a single symbol - a symbol sequence enclosed in square brackets like [A-Za-z] or - a symbol sequence starting with ^ and enclosed in square brackets like [^A-Za-z] (designating the complement of [a-zA-Z]) or - the period (which represents any symbol from the alphabet) Transducer expressions A transducer expression (TE) is recursively defined as follows: - A pair of two symbol ranges separated by a colon is a TE. [a-z]:[a-Z] - A single symbol range like [a-z] is a TE. It is a short form for [a-z]:[a-z]. - Two symbol sequences enclosed in braces and separated by a colon are a TE. {a[bc]}:{def} is equivalent to a:d b:e <>:f | a:d c:e <>:f. - X Y is a TE if X and Y are TEs. (Blanks are ignored unless they are quoted.) - (X) is a TE if X is a TE. - X op is a TE is X is a TE and op is either * (Kleene’s star operator), + (Kleene’s plus operator), or ? (optionality operator) - op X is a TE is X is a TE and op is either ! (negation operator), ^ (target language extraction operator), _ (source language extraction operator), or ^_ (source and target switch operator). - X op Y is a TE is X and Y are TEs and op is either & (conjunction operator), | (disjunction operator), || (composition operator), or - (subtraction operator) - L x op y R is a TE if L and R are TEs, x and y are symbol ranges and op is either => (two-level restriction), <= (two-level coercion), or <=> (two-level restriction and coercion). - X op L__R is a TE if X, L and R are TEs and op is either ^-> (upward replacement), _-> (downward replacement), /-> (leftward replacement) or \-> (rightward replacement). Furthermore, L and R must define automata (i.e. which map their strings onto themselves). These operators correspond to Karttunen’s replace operators. If the arrow is followed by a question mark (?), the replacement becomes optional. - X << l is a TE if X is a TE, and l is either of the form a or the form a:b where a and b are single characters or symbols. The result is a transducer where l was freely inserted into X. The transducer ab << c for instance is equivalent to c*ac*bc*. - X op Y L1__R2, ... , LN__RN is a TE if X,Y, L1 through LN and R1 through RN are TEs, and op is either => (general restriction), <= (general coercion), ^=> (general surface restriction), ^<= (general surface coercion), ^<=> (general surface restriction and coercion), _=> (general deep restriction), _<= (general deep coercion), _<=> (general deep restriction and coercion). (These operators were implemented following a suggestion by Anssi Yli-Jyra.) - "fname" is a TE. The compiler reads the file named fname and turns it into a transducer of the form line1|line2|line3|... where linex is the x-th line of the file. All characters other than : and \ are interpreted literally (i.e. not as operators). This TE is typically used e.g. to read morpheme list from a file. - "<fname>" is a TE. The compiler reads a pre-compiled transducer from the file named fname. This Further Features Comments start with the symbol % and extend up to the end of the line. Blanks are ignored unless they are quoted. Expressions terminate at the end of a line unless the end of line is preceded by a backslash. The command #include "fname" can be used to insert source code from a file named fname. The command RE >> "fname" stores the regular expression RE in the file fname.

Here is an example of a simple transducer program. Assuming that the file "adj-stems" contains the two lines easy late big this transducer will correctly analyse the adjective forms easy, easier, easiest and late, later, and latest. ALPHABET = [a-zA-Z] y:i e:<> <ADJ>:<> $R$ = y<=>i (<ADJ>:<> e) $R2$ = e<=><> (<ADJ>:<> e) $R$ = $R$ & $R2$ $Stems$ = "adj-stems" $S$ = $Stems$ <ADJ> (<pos>:<>|<cmp>:{er}|<sup>:{est}) $S$ || $R$

fst-compilerreturns 0 unless some error occurs.

The compiler gets the operator precedence wrong in case of two-level rules and interprets the expression "ab c<=>d ef" as "a(b c<=>d (ef))". Therefore, you should always surround the left context of two-level rules with parenthesis: (ab) c<=>d (ef)

fst-mor, fst-infl, fst-infl2, fst-infl3, fst-print, fst-compact, fst- parse, fst-compare, fst-compact, fst-lowmem, fst-lattice, fst-train

Helmut Schmid, Institute for Computational Linguistics, University of Stuttgart, Email: schmid@ims.uni-stuttgart.de, This software is available under the GNU Public License. December 2004 fst-compiler(1)