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Commit 7cce2f07 authored by Eckhart Arnold's avatar Eckhart Arnold
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documentation extended

parent a9434c69
......@@ -1183,6 +1183,14 @@ a mistake.) Luckily, although this may sound complicated, in practice it
never is. Unless you grammar is very badly structured, you will hardly
ever make this mistake, an if you do, you will notice soon enough.
Also, there is an important restriction: There is only one §-marker
allowed per named parser. In case you have a long EBFN-expression on the
right hand side of a symbol-definition, where you'd like to use the
§-marker at more than one place, you can, however, always split it into
several expression by introducing new symbols. These symbols, if they
serve no other purpose, can be marked as disposable with the
`@ dispose`-directive (see the description above).
The §-marker has proven to be a very simple means of pinpointing errors
the DSL-code, and I recommend to use it from early on in the process of
developing a new grammar. Plus, the §-marker offers two further benefits,
......@@ -1194,15 +1202,66 @@ only after the grammar has reached a certain amount of maturity, because
changing the grammer ofter requires re-adjusting customized error messages
and resume-clauses as well, which can become tedious.
Customized error message
Customizing error messages
While the error messages produced by the use of the §-marker are often
quite understandable for the engineer designing the grammar of a DSL,
they might not be so for the user the DSL, who might not know the names
of the parsers of the grammar, let alone the expressions of the unnamed
parsers und will therefore not be able to make much sense of a
error-messages that report just these.
parsers und will therefore not always be able to make much sense of
an error-messages that report just these.
In order to customize error messages, the symbol-related directive
`@ SYMBOLNAME_error = CONDITION, ERROR_STRING` is used. The directive's
name consists of the name of a symbol that contains a §-marker and the
appendix `_error`. The directive always takes two arguments, separated
as usual by a comma, of which the first is condition-expression and
the second an error message. The condition can be used to make
the choice of an error-message dependenant on the text following the
point of failure. It can either be
a regular expression or a simple string which must match (or be equal
to in the case of the string) the first part of the text at the
position where the parser defined by the symbol failed to match and
raised an error. Only if the condition matches, the error message
given as the second argument will be emitted. Otherwise, the fallback
error-expression described above ("... expected by parser ...") will
be shown. The empty string `''` can be used as a fallback if the
customized message shall always be emitted, no matter what the
following text looks like.
The error string is a format string that may include any of the
two arguments `{0}` or `{1}` where `{0}` will be replaced by
the name or string representation of the parser that was expected
to match but didn't and `{1}` will be replaced by the first twenty
or so letters of the unmatched rest of the text. Here is a simple
example that could be part of a JSON-parser that is intended to
deliver understandable error-messages::
>>> grammar = '''
... @ string_error = '', 'Illegal character(s) »{1}« in string.'
... string = `"` §characters `"` ~
... characters = { plain | escape }
... plain = /[^"\\\\\\]+/
... escape = /\\\\\\[\\\\/bnrt\\\\\\]/'''
>>> json_string = create_parser(grammar, 'json_string')
>>> print(json_string('"alpha"'))
>>> for e in json_string('"al\\pha"').errors: print(e)
1:4: Error (1010): Illegal character(s) »\pha"...« in string.
1:4: Error (1040): Parser "string" stopped before end, at: \pha" Terminating parser.
Customized error-messages must always be specified in the grammar
before definition of the symbol, they are related to and they can
be stacked. That is, several different error-directives with
different conditions and messages but related to the same symbol
can be specified. The conditions are evaluated in the order the
error-directives appear in the grammar and the error message
of the first matching condition is picked. Therefore, the more
specfic conditions should always be placed first and the more
general or fallback conditions should be placed below these::
Fail-tolerant Parsing
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