unescaped macros

Question

Is there a way to define a macro that does not start with \? for example, I'd like to have -> expand to \rightarrow since I like reading raw LaTeX and $\eps -> 0$ reads much nicer (to me...) than $\eps\rightarrow0$ or even (the solution I have now) $\eps\goto0$.

I guess one could define - as an escape character and then use it to read the next character, if it is a > expand the whole shebang to \rightarrow and if not expand the - to a - and let the next character to its thing...I have no idea if what I just wrote makes any sense, nor how one would do it nor the possible pitfalls and reasons to avoid such behavior....

Ideas? Comments?

Answer 1

You can decide to make - "active" only within math mode, and look ahead for a >.

\documentclass{article}
\mathchardef\minuschar=\mathcode`-\relax
\mathcode`-="8000\relax
\begingroup\makeatletter\lccode`~=`-\lowercase{\endgroup
  \def~{\@ifnextchar{>}{\rightarrow\@gobble}{\minuschar}}}
\begin{document}
We can try $-\int$, $-\sum$, $x->0$, $x- >0$ etc.
\end{document}

When TeX encounters a character with catcode 11 or 12 (letter or other) in math mode, it looks up the character's mathcode. That's a number between "0000 and "8000 (hexadecimal). The value "8000 is treated specially to mean "use the corresponding active character instead". I then defined - to look ahead for a >, and either put a \rightarrow and \@gobble the >, or put the minus character.

Answer 2

The easiest solution, as was observed in one of the comments is to directly type →. Both ConTeXt MkIV and LaTeX + unicode-math give the right output when using the correct unicode input.

If you are using LuaTeX, it is also possible to parse the input and convert -> to →. Here is an example in ConTeXt that uses the translate module

\usemodule[translate]

\translateinput[->][→]
\translateinput[=>][⇒]

\enableinputtranslation

\starttext
$x - y = 0 => x = y$
\stoptext    

Answer 3

There's a package which provides exactly this service, called semantic. It's designed for typsetting programming language semantics, so it provides five things: math ligatures; inference rules (with horizontal lines); T-diagrams (for representing compositions of compilers and interpreters); typesetting for reserved words; and default shorthands. Obviously, we care about the two in bold.

Math ligatures let you do precisely the following:

\documentclass{minimal}

\usepackage[ligature]{semantic}

\mathlig{->}{\rightarrow}

\begin{document}
  \[ a_{>} -> b_{-} \]
\end{document}

This will typeset just the same as writing \[ a_{>} \rightarrow b_{-} \].

You can define ligatures to be arbitrary sequences of characters (for precise details, check the documentation), and if you specify the [shorthand] option to the package instead, it will define a default set (again, the documentation has the list).

As for limitations, there's only one that I've run into: having b_- alone doesn't work (whether or not it's at the end of math mode), because - is an initial character of a ligature (for instance, a_> works fine). I have no clue why this is. I would also guess that redefining - as an active character would break, but I've never tried. Honestly, I've never run into it before, but now that someone mentioned similar things I thought I'd check.

Answer 4

Just for the record,

\usepackage{newunicodechar}
\newunicodechar{→}{\to}

allows for

$x→0$

with pdflatex (the file must be coded in UTF-8 and \usepackage[utf8]{inputenc} must be used, not utf8x). It also works with xelatex or lualatex (without unicode-math, of course, otherwise it would be nonsense).

Answer 5

It's certainly possible to assign a character other than \ (backslash) to function as the start a control word (macro) in TeX. But, what would you gain? Even if all macros started with a ~ (tilde) character instead of a \, it wouldn't affect the length of the strings used to denote various commands, would it?

By the way, \to is a synonym for \rightarrow. Is \to short enough (and readable enough) for you? :-)

Sort-of Lengthy Addendum To make the specific character combination -> into a "command" that typesets the equivalent of \to in math mode, you'd have to make the first character, -, "active" in TeX jargon and then define a macro that reads ahead to the next character; if that character is a >, you'd tell TeX to typeset \rightarrow, and if it's not, you'd instruct TeX to place the - character and the following character back into the input token stream.

Now, back to the question of how to make the "instruction" -> execute the instruction \rightarrow. Study the code in the MWE below; the \catcode command makes the - active, and the following line provides the definition of the command. Note that we can't just place - back into the token stream, as TeX would find itself in an ever and deeper loop trying to resolve what to do with the command -. Instead, I use the command \char command to place a dash back into the token stream.

\documentclass{article}
\catcode`\-=\active
\def-#1{\ifx>#1 \ensuremath{\rightarrow} \else \char`-#1 \fi}

\begin{document}
-> \quad -+ \quad -, \quad $-a$ \quad 5--9 \quad $x->0$
\end{document}

(Well, I'm sure I've left out some eventuality that'll make this code break down in some circumstances; I'll let our fellow TeX-SX hounds point out the mistakes...)

Answer 6

You can always make - an active character and assign a macro to it. Below I use a "delimited" macro to make -> produce the "\rightarrow" token. The problem is - is redefined everywhere. You normally need to have some sort of scoping macro that turns active characters on and off:

\documentclass{article}
\begin{document}
\catcode `-=\active\def->{\ensuremath{\rightarrow}}
hello ->.
\end{document}

Search for "catcode" and "active characters" to learn more.

Answer 7

Since you mentioned reading sources, I can suggest an alternative: set up your editor to display \rightarrow as →.

Emacs 24.5 with AucTeX does this if you enable prettify-symbols-mode:

(left: without prettify-symbols-mode; right: with prettify-symbols-mode. Same text in both buffers)