A syntax error occurs when data are read from some external source
but have an improper format or cannot be processed for some other reason.
This category mainly applies to read/1 and its variants.
The exception code associated with a syntax error is
syntax_error(Goal, Position, Message, Left, Right)
where Goal is the goal in question, Position identifies the position in the stream where reading started, and Message describes the error. Left and right are lists of tokens before and after the error, respectively.
Note that the Position is where reading started, not where the error is.
read/1 does two things. First, it reads a sequence of characters from the
current input stream up to and including a clause terminator, or the end of
file marker, whichever comes first. Then it attempts to parse the sequence
of characters as a Prolog term. If the parse is unsuccessful, a syntax error
occurs. Thus, in the case of syntax errors, read/1 disobeys the normal
rule that predicates should detect and report errors before they perform any
side-effects, because the side-effect of reading the characters has been done.
A syntax error does not necessarily cause an exception to be raised. The behavior can be controlled via a Prolog flag as follows:
prolog_flag(syntax_errors, quiet)
read/1
just quietly fails.
prolog_flag(syntax_errors, dec10)
user_error, and
the read is repeated. This is the default for the sake of
compatibility with earlier releases.
prolog_flag(syntax_errors, fail)
user_error, and the
read then fails.
prolog_flag(syntax_errors, error)