Options

Scripts > Productions > SKIP > Options

There are three options for the SKIP symbol which control the treatment of possible conflicts. Two of this can get predefined in the project options. If a not predefined option shall be used, then this can be expressed by appending a parameter to "SKIP". E.g.

SKIP[ F ] 

1. no failure alternative

If the use of global scanners is set in the project options, it can happen that at the current position of the source text a token which isn't expected in the grammar is recognized. For example regular expression for identifiers is defined in many projects. If now

SKIP "Welt"

is used to parse

Hello world

"hello" would be interpreted as an identifier. According to the scanner algorithm, SKIP wouldn't be tested. Instead  an error message would be produced. If no failure alternatives are permitted, however, "hello" becomes skipped and therefore the complete text would be parsed correctly.

Within a production this option can be set explicitly with the parameter NFA (no failure alternative):

SKIP[ NFA ] 

There is a problem with hidden alternatives at this option, though. Example:

( ID ";"* )* SKIP

If a text starts with a semicolon, then it is skipped with SKIP. If the text starts with an identifier on which a semicolon follows, then the identifier is recognized correctly. However, the semicolon is skipped again. Although it is recognized at first it is not judged to be an alternative to SKIP. In this case the strict option 3 would be adequate for SKIP (see below)

2. allow skip destinations at the current position // experimentally

Ein ähnlicher Konflikt wie eben beschrieben entsteht, wenn schon an der aktuellen Position des Quelltextes ein Token erkannt wird, das als Ziel des SKIP-Knotens gesetzt ist.

A similar conflict as just described arises if a token which is the destination of  SKIP, is already recognized at the current position of the source text.

SKIP ID

The word "Hello" in the text:

Hello world

then would be recognized as identifier. The option described above, not to allow failure alternatives, would not help, since the skip destinations aren't part of these alternatives. If they were added, an expression like

SKIP? ID

would become senseless. SKIP would always match on the text (if not at the end with no following identifier). So it is to recommend to amend the grammar correspondingly: With

SKIP? ID+

"Hello world" is parsed correctly. Experimentally there is the NF-option (no failure)

SKIP[ NF ] // experimentally

This option possibly will be no longer available in future TETRA versions, if not desired by users.

3. strict generation of errors

Errors were always caused up to TextTransformer 1.5.0 in the cases represented above. This option forces the programmer to consider and to treat conflicts explicitly. The possibility of making SKIP optionally was already mentioned. One other possibility would be:

( SKIP | ID ) ID

A strict generation of errors can be forced in a production with the F parameter:

SKIP[ F ] 

Note:

It shall be pointed out again that the mentioned conflict possibilities result from the  option to use global scanners. A conflict with literals only can arise if also the option to test all literals is set and a literal is an immediate alternative to SKIP. Otherwise literals aren't tested at all.

This page belongs to the TextTransformer Documentation