doc-src/System/fonts.tex

under construction;

\chapter{Fonts and character encodings}

With print modes being now available in Isabelle, variant forms of
output have become very easy. As the canonical application of this
feature, {\Pure} and major object-logics (\FOL, \ZF, \HOL, \HOLCF)
support optional input and output of nice mathematical symbols as an
built-in option.

Symbolic output is enabled by activating the \ttindex{symbols} print
mode. User interfaces (e.g.\ \texttt{isa-xterm}, see
\S\ref{sec:isa-xterm}) usually do this already by default.

\medskip Displaying non-standard characters requires special screen
fonts, of course. The \texttt{installfonts} utility takes care of
this, see \S\ref{sec:tool-installfonts}. Furthermore, some {\ML}
systems disallow non-\textsc{ascii} characters in literal strings.
This problem is avoided by the \texttt{symbolinput} filter (see
\S\ref{sec:tool-symbolinput}).

Both of these are invoked transparently in normal operation. So one
does not actually have to read the explanations below, unless
something fails to work.


\section{Telling X11 about the Isabelle fonts --- \texttt{isatool installfonts}}
\label{sec:tool-installfonts}

The \tooldx{installfonts} utility ensures that your currently running
X11 display server (as determined by the \texttt{DISPLAY} environment
variable) knows about the Isabelle fonts. Its usage is:
\begin{ttbox}
Usage: isatool installfonts

  Install the isabelle fonts into your X11 server.
  (May be safely called repeatedly.)
\end{ttbox}
Note that this need not be called manually under normal circumstances
--- user interfaces depending on the Isabelle fonts usually invoke
\texttt{installfonts} automatically.

\medskip As simple as this might appear to be, it is not! X11 fonts
are a surprisingly complicated matter. Depending on your network
structure, fonts might have to be installed differently. This has to
be specified via the \settdx{ISABELLE_INSTALLFONTS} variable in your
local settings.

\medskip In the simplest situation, X11 is used only locally, i.e.\ 
the client program (Isabelle) and the display server are run on the
same machine. In this case, most X11 display servers should be happy
by being about the directory where the fonts reside as follows:
\begin{ttbox}
ISABELLE_INSTALLFONTS="xset fp+ $ISABELLE_HOME/lib/fonts; xset fp rehash"
\end{ttbox}
The same also works for remote X11 sessions in a somewhat homogeneous
network, where the X11 display machine mounts the Isabelle
distribution under the same name as the client side.

\medskip Above method fails, though, if the display machine does have
the font files at the same location as the client. In case your server
is a full workstation with its own file system, you could in principle
just copy the fonts there and do an appropriate \texttt{xset~fp+}
manually before running the Isabelle interface. This is very awkward,
of course. It is even \emph{impossible} for proper X terminals that do
not have their own file system.

A much better solution to this problem is to have an X11 \emph{font
  server} offer the Isabelle fonts to any X11 display on the network.
There is already a font server running at Munich. So in case you have
a sensible Internet connection, you may plug attach yourself as
follows:
\begin{ttbox}
ISABELLE_INSTALLFONTS="xset fp+ tcp/isafonts.informatik.tu-muenchen.de:7200"
\end{ttbox}

\medskip In the (rare?) case that neither local fonts work, nor
accessing our world-wide font service is practical, it might be best
to start your own in-house font service. This is in principle quite
easy to setup. The program is called \texttt{xfs} (or just
\texttt{fs)}, see the \texttt{man} pages of your system. There is an
example configuration available in the \texttt{lib/fontserver}
directory of the Isabelle distribution.



\section{Filtering non-ASCII characters --- \texttt{isatool symbolinput}}
\label{sec:tool-symbolinput}

Processing non-\textsc{ascii} text is notoriously difficult.  Some
{\ML} systems reject character codes outside the range 32--127 as part
of literal string constants. In order to circumvent such restrictions,
Isabelle employs a general notation where glyphs are referred by some
symbolic name instead of their actual encoding. Its general form is
\texttt{$\backslash$<$charname$>}.

The \tooldx{symbolinput} utility converts a input stream encoded
according to the standard Isabelle font layout into pure
\textsc{ascii} text. There is no usage --- \texttt{symbolinput} just
works from standard input to standard output, without any options
available.

\medskip For example, the non-\textsc{ascii} input \texttt{"A $\land$
  B $\lor$ C"} is replaced by \texttt{"A $\backslash\backslash$<and> B
  $\backslash\backslash$<or> C"}.

\medskip In many cases, it might be wise not to rely on symbolic
characters and avoid non-\textsc{ascii} text in files altogether. Then
symbolic syntax would be really optional, with always suitable
\texttt{ascii} representations available. In theory definitions
symbols appear only in mixfix annotations --- using the
\texttt{$\backslash$<$charname$>} form, proof scripts are just left in
plain \texttt{ascii}.

Thus users with \texttt{ascii}-only facilities will still be able to
read your files.


%FIXME not yet
%\section{ --- \texttt{isatool showsymbols}}
%
%\begin{ttbox}
%\end{ttbox}