# HaTeX User's Guide

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## Revision as of 16:47, 15 September 2013

## Contents |

## 1 Preface

### 1.1 Introduction

If you are here because you want to learn more about HaTeX, or just feel
curious, you are in the right place. First of all, note that this guide is addressed to that
people that already knows the basics of both Haskell and LaTeX. Otherwise, try to learn first
a bit of these languages (both are quite useful learnings). To learn Haskell, though I guess
you already learned it since you are reading these lines, go to the Haskell web [1]
and search for some tutorials or books. To learn LaTeX, you can start with
*The not so short introduction to LaTeX* [2].

The HaTeX library aspires to be the tool that Haskellers could want to make their LaTeX things without exit of their language (we understand that is difficult to leave Haskell after the first date), trying to be the most comprehensive and well done as possible. Do you think, anyway, that something could be done better? Perhaps something is lacked? Go then to the HaTeX mailing list [3] and leave your complain without mercy! Or, in the case you are a GitHub user, say your word in the issue list [4] or, to be awesome, make yourself a patch and send a pull request. This is the great thing about open source projects!

### 1.2 What is HaTeX?

Before we explain *how* HaTeX works, it is convenient to say *what* actually HaTeX is.

*HaTeX is a Haskell library that provides functions to create, manipulate and parse LaTeX code.*

People often says that *HaTeX is a LaTeX DSL*. With it you can enjoy all the advantages
you already have in Haskell while creating LaTeX documents. A common purpose is to
automatize the creation of such documents, perhaps from a source data in Haskell.
A more exotic one is to render chess tables. Possibilities are in a wide range.
The idea is the following: if you can do it with LaTeX, you can do it with HaTeX,
but adding all the Haskell features.

## 2 Basics

Through this section you will learn the basics of HaTeX. Essentially, *how* it works.

### 2.1 The Monoid class

If you are already familiar with theA *monoid* in Mathematics is an algebraic structure consisting of a set of objects with
an operation between them, being this operation *associative* and with a *neutral element*.
Phew! But what is the meaning of this? By *associative* we mean that, if you have three elements
*a*, *b* and *c*, then *a* * (*b* * *c*) = (*a* * *b*) * *c*. A *neutral element* is the one that does not worth to operate with,
because it does nothing! To say, *e* is a *neutral element* if *e* * *a* = *a* * *e* = *a*, given any object *a*.
As an example, you may take the *real numbers* as objects and the ordinary multiplication as operation.

class Monoid m where mempty :: m mappend :: m -> m -> m mconcat :: [m] -> m

The names of the methods may seem insuitable, but they correspond to an example of monoid:

the lists with the appendingxs ++ [] = [] ++ xs = xs

### 2.2 LaTeX blocks

Suppose we have a well-formed^{1}
piece of LaTeX code, call it *a*.

*a*can be seen as a Haskell expression

**block**. What happens if we append, by juxtaposition, two

a totally empty code is a well-formed piece of LaTeX code, we can speak about the empty block.

And, as the reader may notice, these blocks with its appending form a monoid. Namely,to collapse them all in a single block.

### 2.3 Creating blocks

We have now a universe of blocks forming a monoid. What we need now is a way to create these blocks.

As we said, a block is the representation of a well-formed piece of LaTeX code. Let^{2}. Since this is a constant expression, it has a constant value in Haskell, named

this value will generate the desired block.

Other LaTeX expressions depend on a given argument. For examplea number. How we deal with this? As you expect, with functions. We can create blocks that depend on values with functions that take these values as arguments, where these arguments can be

blocks as well. For instance, we have the functionlinespread :: Float -> LaTeX

title :: LaTeX -> LaTeX

**to create blocks and combine them**.

Once you have your final block ready, you will be able to create its corresponding LaTeX code
(we will see how later). Note that for every block there is a LaTeX code, but not for every code
there is a block, because a malformed (in the sense of the negation of our well-formed concept) code
has **not** a block in correspondence.
This fact has a practical consequence: **we cannot create malformed LaTeX code**. *And that's a good deal!*

#### 2.3.1 From strings

Inserting text in a LaTeX document is a constant task. You can create a block with text given

an arbitraryclass IsString a where fromString :: String -> a

HaTeX takes care and avoids them replacing each reserved character with a command which

output looks like the original character. For example, the backslash*Overloaded Strings*extension.

This one is similar to the *Overloaded Numbers* Haskell feature, which translates the number

*foo*as content. Quite handy! We will assume the

#### 2.3.2 More blocks

There is a lot of functions for create blocks. In fact, we can say that this is the main purpose of the library. LaTeX has a lot of commands, in order to set font attributes, create tables, insert graphics, include mathematical symbols, etc. So HaTeX have a function for each command defined in LaTeX (to tell the truth, only for a small subset). Please, go to the API documentation to read about particular functions. Build it locally or find it in Hackage: http://hackage.haskell.org/package/HaTeX.

You will find the class constraint### 2.4 Putting blocks together

Once you have the blocks, as we said before, you need to append them. The^{3}, is the block with

as follows:

mconcat [ "I can see a " , textbf "rainbow" , " in the blue " , textit "sky" , "." ]

This is the last step in our LaTeX document creation. When we have our final

LaTeX blockthe form of its correspondent LaTeX code.

Say we have the next definition:

short = documentclass [] article <> title "A short message" <> author "John Short" <> document (maketitle <> "This is all.")

in the current working directory (line breaks added for easier visualization):

\documentclass{article} \title{A short message} \author{John Short} \begin{document} \maketitle{} This is all \end{document}

output to dvi or pdf.

The functionrenderFile :: Render a => FilePath -> a -> IO ()

class Render a where render :: a -> Text

These instances are useful for creating blocks from other values. With the function

**be careful!**Because

**not**escape reserved characters.

### 2.5 Try yourself

As always, the best way to learn something well is to try it by yourself. Since to see code examples can give you a great help, HaTeX comes with several examples where you can see by yourself how to get the work done.

The API reference is also a good point to keep in mind. Descriptions of functions make you know how exactly they works. And, when they are not present, function names with type signatures may be very helpful and descriptive.

## 3 LaTeX blocks and the Writer monad

### 3.1 The Writer Monad

Fixed a monoidand elements from other types. Thus, the Haskell declaration is as follows^{4}:

data W m a = W m a

of the monoid.

inject :: Monoid m => a -> W m a inject a = W mempty a

instance Functor (W m) where fmap f (W m a) = W m (f a)

join :: Monoid m => W m (W m a) -> W m a join (W m (W m' a)) = W (mappend m m') a

respectively. In practice, this is because they act equal. Indeed, they are equal if we forget the

instance Monoid m => Monad (W m) where return = inject w >>= f = join (fmap f w)

What we have done here is to hide in a monad a monoid with all its operations. We have created a

machine that operates monoid values. To insert a value into the machine we need thefunction:

tell :: m -> W m () tell m = W m ()

execute :: W m a -> m execute (W m a) = m

instance Monoid Int where mempty = 0 mappend = (+) example :: Int example = execute $ do tell 1 tell 2 tell 3 tell 4

example :: Int example = execute $ mapM_ tell [ 1 .. 4 ]

type LaTeXW = W LaTeX

example :: LaTeX example = execute $ do tell $ documentclass [] article tell $ author "Monads lover" tell $ title "LaTeX and the Writer Monad"

author' :: LaTeXW a -> LaTeXW () author' = tell . author . execute

disappears.

example :: LaTeX example = execute $ do documentclass' [] article author' "Monads lover" title' "LaTeX and the Writer Monad"

### 3.2 Composing monads

To add flexibility to HaTeX, the writer monad explained above is defined as a monad transformer,

namedThe first change is in type signatures. We need to carry an inner monad in every type.

foo :: Monad m => LaTeXT m a

type LaTeXW = LaTeXT M foo :: LaTeXW a

of our inner monad at any time. For example, suppose we want to output some code we have in
the file *foo.hs*. Instead of copy all its content, or read and carry it as an argument along the code,

type LaTeXIO = LaTeXT IO readCode :: FilePath -> LaTeXIO () readCode fp = lift (readFileTex fp) >>= verbatim . raw example :: LaTeXIO () example = do "This is the code I wrote this morning:" readCode "foo.hs" "It was a funny exercise."

these features, it is enough to use the Identity monad.

type LaTeXW = LaTeXT Identity

## 4 The LaTeXC class

HaTeX has two different interfaces. One uses blocks asfunction definitions^{5}
or to have a typeclass which unifies both interfaces. Since duplicate definitions is a hard work

^{6}, we took the second alternative and defined the

typeclass. This way, we have both interfaces with a single import, without being worry about maintaining duplicated code. The cost is to have class constraints in type signatures. But these constraints are only required in the package. At the user level, you choose your interface and write type signatures in consequence.

## 5 Packages

LaTeX, in addition to its predefined commands, has a big number of packages that increase its power. HaTeX functions for some of these packages are defined in separated modules, one module per package. This way, you can import only those functions you actually need. Some of these modules are below explained.

### 5.1 Inputenc

This package is of vital importance if you use non-ASCII characters in your document.
For example, if my name is *Ángela*, the *Á* character will not appear correctly in the

import Text.LaTeX.Base import Text.LaTeX.Packages.Inputenc thePreamble :: LaTeX thePreamble = documentclass [] article <> usepackage [utf8] inputenc <> author "Ángela" <> title "Issues with non-ASCII characters"

### 5.2 Graphicx

With thefunction.

includegraphics :: LaTeXC l => [IGOption] -> FilePath -> l

height, scaling or rotation. See the API documentation for details.

## 6 Epilogue

### 6.1 Notes about this guide

**This guide is not static**. It will certainly be changed with the time.
Any reader can participate as a writer since the guide is itself open source (and
written in Haskell!). The source repository can be reached at:
https://github.com/Daniel-Diaz/hatex-guide. Read more detailed instructions in the
README file.

If you think there is something unclear, something hard to understand, please, report it.

### 6.2 Notes from the author

I would like to end this guide saying thanks to all the people that has been interested
in HaTeX somehow, especially to those who contributed to it with patches, opinions
or bug reports. **Thanks**.

## 7 Footnotes

^{1}: With *well-formed* we mean that all braces, environments, math expressions, ... are closed.

^{2}: Please, note that the

**not**the same that the LaTeX expression. The former

is a Haskell value, not the LaTeX code itself.

^{3}: From

**GHC 7.4**,

versions of GHC, HaTeX exports the synonym.

^{4}: Some authors write it using tuples, like this:

^{5}: This was the approach taken in HaTeX 3 until the version 3.3, where the

^{6}: In fact, we had a problem with HaTeX-meta, the program that automatically generated the duplicated functions.
The problem was described in a blog post: http://deltadiaz.blogspot.com.es/2012/04/hatex-trees-and-problems.html.