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\section{library(readutil): Read utilities}

\label{sec:readutil}

\begin{tags}
\mtag{See also}- \file{library(pure_input)} allows for processing files with DCGs. \\- \file{library(lazy_lists)} for creating lazy lists from input.
\end{tags}

This library provides some commonly used reading predicates. As these
predicates have proven to be time-critical in some applications we moved
them to C. For compatibility as well as to reduce system dependency, we
link the foreign code at runtime and fallback to the Prolog
implementation if the shared object cannot be found.\vspace{0.7cm}

\begin{description}
    \predicate[det]{read_line_to_codes}{2}{+Stream, -Line:codes}
Read the next line of input from \arg{Stream}. Unify content of the lines
as a list of character codes with \arg{Line} \textit{after} the line has been
read. A line is ended by a newline character or end-of-file. Unlike
\predref{read_line_to_codes}{3}, this predicate removes a trailing newline
character.

    \predicate[det]{read_line_to_codes}{3}{+Stream, -Line, ?Tail}
Difference-list version to read an input line to a list of character
codes. Reading stops at the newline or end-of-file character, but
unlike \predref{read_line_to_codes}{2}, the newline is retained in the output.
This predicate is especially useful for reading a block of lines up
to some delimiter. The following example reads an HTTP header ended
by a blank line:

\begin{code}
read_header_data(Stream, Header) :-
    read_line_to_codes(Stream, Header, Tail),
    read_header_data(Header, Stream, Tail).

read_header_data("\r\n", _, _) :- !.
read_header_data("\n", _, _) :- !.
read_header_data("", _, _) :- !.
read_header_data(_, Stream, Tail) :-
    read_line_to_codes(Stream, Tail, NewTail),
    read_header_data(Tail, Stream, NewTail).
\end{code}

    \predicate[det]{read_line_to_string}{2}{+Stream, -String}
Read the next line from \arg{Stream} into \arg{String}. \arg{String} does not contain
the line terminator. \arg{String} is unified with the \textit{atom} \verb$end_of_file$
if the end of the file is reached.

\begin{tags}
    \tag{See also}
\predref{read_string}{5} can be used to read lines with separated
records without creating intermediate strings.
\end{tags}

    \predicate[det]{read_stream_to_codes}{2}{+Stream, -Codes}
\nodescription
    \predicate[det]{read_stream_to_codes}{3}{+Stream, -Codes, ?Tail}
Read input from \arg{Stream} to a list of character codes. The version
\predref{read_stream_to_codes}{3} creates a difference-list.

    \predicate[det]{read_file_to_codes}{3}{+Spec, -Codes, +Options}
Read the file \arg{Spec} into a list of \arg{Codes}. \arg{Options} is split into
options for \predref{absolute_file_name}{3} and \predref{open}{4}. In addition, the
following option is provided:

\begin{description}
    \termitem{tail}{?Tail}
Read the data into a \textit{difference list} \arg{Codes}\Sneg{}\arg{Tail}.
\end{description}

\begin{tags}
    \tag{See also}
\predref{phrase_from_file}{3} and \predref{read_file_to_string}{3}.
\end{tags}

    \predicate[det]{read_file_to_string}{3}{+Spec, -String, +Options}
Read the file \arg{Spec} into a the string \arg{String}. \arg{Options} is split
into options for \predref{absolute_file_name}{3} and \predref{open}{4}.

\begin{tags}
    \tag{See also}
\predref{phrase_from_file}{3} and \predref{read_file_to_codes}{3}.
\end{tags}

    \predicate[det]{read_file_to_terms}{3}{+Spec, -Terms, +Options}
Read the file \arg{Spec} into a list of terms. \arg{Options} is split over
\predref{absolute_file_name}{3}, \predref{open}{4} and \predref{read_term}{3}. In addition, the
following option is processed:

\begin{description}
    \termitem{tail}{?Tail}
If present, \arg{Terms}\Sneg{}\arg{Tail} forms a \textit{difference list}.
\end{description}

Note that the \textit{output} options of \predref{read_term}{3}, such as
\verb$variable_names$ or \verb$subterm_positions$ will cause
\predref{read_file_to_terms}{3} to fail if \arg{Spec} contains multiple terms
because the values for the different terms will not unify.
\end{description}