# doctemplates

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This is the text templating system used by pandoc.  Its basic function is to
fill variables in a template.  Variables are provided by a "context."
Any instance of the `ToContext` typeclass (such as an aeson `Value`)
can serve as the context, or a `Context` value can be constructed manually.

Control structures are provided to test whether a variable has a non-blank
value and to iterate over the items of a list.  Partials---that is,
subtemplates defined in different files---are supported.  Pipes
can be used to transform the values of variables or partials.
The provided pipes make it possible to do list enumeration and
tabular layout in templates.

Templates are rendered to a doclayout `Doc` (which is polymorphic
in the underlying string type). If `Doc` values are used in the
context, rendered documents will be able to wrap flexibly on breaking
spaces.  This feature makes doctemplates more suitable than other
template engines for plain-text formats (like Markdown).

Unlike the various HTML-centered template engines, doctemplates is
output-format agnostic, so no automatic escaping is done on interpolated
values.  Values are assumed to be escaped properly in the Context.

## Example of use

``` haskell
{-# LANGUAGE OverloadedStrings #-}
import Data.Text (Text)
import qualified Data.Text.IO as T
import Data.Aeson
import Text.DocTemplates
import Text.DocLayout (render)

data Employee = Employee { firstName :: String
                         , lastName  :: String
                         , salary    :: Maybe Int }
instance ToJSON Employee where
  toJSON e = object [ "name" .= object [ "first" .= firstName e
                                       , "last"  .= lastName e ]
                    , "salary" .= salary e ]

template :: Text
template = "$for(employee)$Hi, $employee.name.first$. $if(employee.salary)$You make $employee.salary$.$else$No salary data.$endif$$sep$\n$endfor$"

main :: IO ()
main = do
  res <- compileTemplate "mytemplate.txt" template
  case res of
         Left e    -> error e
         Right t   -> T.putStrLn $ render Nothing $ renderTemplate t $ object
                        ["employee" .=
                          [ Employee "John" "Doe" Nothing
                          , Employee "Omar" "Smith" (Just 30000)
                          , Employee "Sara" "Chen" (Just 60000) ]
                        ]
```

## Delimiters

To mark variables and control structures in the template,
either `$`...`$` or `${`...`}` may be used as delimiters.
The styles may also be mixed in the same template, but the
opening and closing delimiter must match in each case.  The
opening delimiter may be followed by one or more spaces
or tabs, which will be ignored. The closing delimiter may
be followed by one or more spaces or tabs, which will be
ignored.

To include a literal `$` in the document, use `$$`.

## Comments

Anything between the sequence `$--` and the end of the
line will be treated as a comment and omitted from the output.

## Interpolated variables

A slot for an interpolated variable is a variable name surrounded
by matched delimiters.  Variable names must begin with a letter
and can contain letters, numbers, `_`, `-`, and `.`.  The
keywords `it`, `if`, `else`, `endif`, `for`, `sep`, and `endfor` may
not be used as variable names. Examples:

```
$foo$
$foo.bar.baz$
$foo_bar.baz-bim$
$ foo $
${foo}
${foo.bar.baz}
${foo_bar.baz-bim}
${ foo }
```

The values of variables are determined by the `Context` that is
passed as a parameter to `renderTemplate`.  So, for example,
`title` will return the value of the `title` field, and
`employee.salary` will return the value of the `salary` field
of the object that is the value of the `employee` field.

- If the value of the variable is simple value, it will be
  rendered verbatim.  (Note that no escaping is done;
  the assumption is that the calling program will escape
  the strings appropriately for the output format.)
- If the value of the variable is a boolean value, it
  will be rendered as `true` if true, or as empty if false.
- If the value is a list, the values will be concatenated.
- If the value is a map, the string `true` will be rendered.
- Every other value will be rendered as the empty string.

When a `Context` is derived from an aeson (JSON) `Value`,
the following conversions are done:

- If the value is a number, it will be rendered as an
  integer if possible, otherwise as a floating-point number.


## Conditionals

A conditional begins with `if(variable)` (enclosed in
matched delimiters) and ends with `endif` (enclosed in matched
delimiters).  It may optionally contain an `else` (enclosed in
matched delimiters).  The `if` section is used if
`variable` has a true value, otherwise the `else`
section is used (if present).  The following values
count as true:

- any map
- any array containing at least one true value
- any nonempty string (even `false`)
- boolean True

Examples:

```
$if(foo)$bar$endif$

$if(foo)$
  $foo$
$endif$

$if(foo)$
part one
$else$
part two
$endif$

${if(foo)}bar${endif}

${if(foo)}
  ${foo}
${endif}

${if(foo)}
${ foo.bar }
${else}
no foo!
${endif}
```

The keyword `elseif` may be used to simplify complex nested
conditionals.  Thus

```
$if(foo)$
XXX
$elseif(bar)$
YYY
$else$
ZZZ
$endif$
```

is equivalent to

```
$if(foo)$
XXX
$else$
$if(bar)$
YYY
$else$
ZZZ
$endif$
$endif$
```

## For loops

A for loop begins with `for(variable)` (enclosed in
matched delimiters) and ends with `endfor` (enclosed in matched
delimiters).

- If `variable` is an array, the material inside the loop will
  be evaluated repeatedly, with `variable` being set to each
  value of the array in turn, and concatenated.
- If `variable` is a map, the material inside will be set to
  the map.
- If the value of the associated variable is not an array or
  a map, a single iteration will be performed on its value.

Examples:

```
$for(foo)$$foo$$sep$, $endfor$

$for(foo)$
  - $foo.last$, $foo.first$
$endfor$

${ for(foo.bar) }
  - ${ foo.bar.last }, ${ foo.bar.first }
${ endfor }

$for(mymap)$
$it.name$: $it.office$
$endfor$
```

You may optionally specify a separator between consecutive
values using `sep` (enclosed in matched delimiters).  The
material between `sep` and the `endfor` is the separator.

```
${ for(foo) }${ foo }${ sep }, ${ endfor }
```

Instead of using `variable` inside the loop, the special
anaphoric keyword `it` may be used.

```
${ for(foo.bar) }
  - ${ it.last }, ${ it.first }
${ endfor }
```

## Partials

Partials (subtemplates stored in different files) may be
included using the syntax

```
${ boilerplate() }
```

The partials are obtained using `getPartial` from
the `TemplateMonad` class.  This may be implemented
differently in different monads.  The path passed
to `getPartial` is computed on the basis of the
original template path (a parameter to `compileTemplate`)
and the partial's name.  The partial's name is substituted
for the *base name* of the original template path
(leaving the original template's extension), unless
the partial has an explicit extension, in which case
this is kept.  So, with the `TemplateMonad` instance
for IO, partials will be sought in the directory
containing the main template, and will be assumed
to have the extension of the main template.

Partials may optionally be applied to variables using
a colon:

```
${ date:fancy() }

${ articles:bibentry() }
```

If `articles` is an array, this will iterate over its
values, applying the partial `bibentry()` to each one.  So the
second example above is equivalent to

```
${ for(articles) }
${ it:bibentry() }
${ endfor }
```

Note that the anaphoric keyword `it` must be used when
iterating over partials.  In the above examples,
the `bibentry` partial should contain `it.title`
(and so on) instead of `articles.title`.

Final newlines are omitted from included partials.

Partials may include other partials.  If you exceed
a nesting level of 50, though, in resolving partials,
the literal `(loop)` will be returned, to avoid infinite loops.

A separator between values of an array may be specified
in square brackets, immediately after the variable name
or partial:

```
${months[, ]}$

${articles:bibentry()[; ]$
```

The separator in this case is literal and (unlike with `sep`
in an explicit `for` loop) cannot contain interpolated
variables or other template directives.

## Nesting

To ensure that content is "nested," that is, subsequent lines
indented, use the `^` directive:

```
$item.number$  $^$$item.description$ ($item.price$)
```

In this example, if `item.description` has multiple lines,
they will all be indented to line up with the first line:

```
00123  A fine bottle of 18-year old
       Oban whiskey. ($148)
```

To nest multiple lines to the same level, align them
with the `^` directive in the template. For example:

```
$item.number$  $^$$item.description$ ($item.price$)
               (Available til $item.sellby$.)
```

will produce

```
00123  A fine bottle of 18-year old
       Oban whiskey. ($148)
       (Available til March 30, 2020.)
```

If a variable occurs by itself on a line, preceded by whitespace
and not followed by further text or directives on the same line,
and the variable's value contains multiple lines, it will be
nested automatically.

## Breakable spaces

When rendering to a `Doc`, a distinction can be made between
breakable and unbreakable spaces.  Normally, spaces in the
template itself (as opposed to values of the interpolated
variables) are not breakable, but they can be made breakable
in part of the template by using the `~` keyword (ended
with another `~`).

```
$~$This long line may break if the document is rendered
with a short line length.$~$
```

The `~` keyword has no effect when rendering to `Text`
or `String`.

## Pipes

A pipe transforms the value of a variable or partial.  Pipes
are specified using a slash (`/`) between the variable name (or
partial) and the pipe name.  Example:

```
$for(name)$
$name/uppercase$
$endfor$

$for(metadata/pairs)$
- $it.key$: $it.value$
$endfor$

$employee:name()/uppercase$
```

Pipes may be chained:

```
$for(employees/pairs)$
$it.key/alpha/uppercase$. $it.name$
$endfor$
```

Some pipes take parameters:

```
|----------------------|------------|
$for(employee)$
$it.name.first/uppercase/left 20 "| "$$it.name.salary/right 10 " | " " |"$
$endfor$
|----------------------|------------|
```

Currently the following pipes are predefined:

- `pairs`:  Converts a map or array to an array of maps,
  each with `key` and `value` fields.  If the original
  value was an array, the `key` will be the array index,
  starting with 1.

- `first`: Returns the first value of an array, if
  applied to a non-empty array; otherwise returns
  the original value.

- `last`: Returns the last value of an array, if
  applied to a non-empty array; otherwise returns
  the original value.

- `rest`: Returns all but the first value of an array, if
  applied to a non-empty array; otherwise returns
  the original value.

- `allbutlast`: Returns all but the last value of an array, if
  applied to a non-empty array; otherwise returns
  the original value.

- `uppercase`:  Converts text to uppercase.

- `lowercase`:  Converts text to lowercase.

- `length`:  Returns the length of the value:  number
  of characters for a textual value, number of elements
  for a map or array.

- `reverse`:  Reverses a textual value or array,
  and has no effect on other values.

- `chomp`:  Removes trailing newlines (and breakable space).

- `nowrap`:  Disables line wrapping on breakable spaces.

- `alpha`:  Converts textual values that can be
  read as an integer into lowercase alphabetic
  characters `a..z` (mod 26). This can be used to get lettered
  enumeration from array indices.  To get uppercase
  letters, chain with `uppercase`.

- `roman`:  Converts textual values that can be
  read as an integer into lowercase roman numerials.
  This can be used to get lettered enumeration from array indices.
  To get uppercase roman, chain with `uppercase`.

- `left n "leftborder" "rightborder"`:  Renders a textual value
  in a block of width `n`, aligned to the left, with an optional
  left and right border.  Has no effect on other values. This
  can be used to align material in tables.  Widths are positive
  integers indicating the number of characters.  Borders
  are strings inside double quotes; literal `"` and `\` characters
  must be backslash-escaped.

- `right n "leftborder" "rightborder"`:  Renders a textual value
  in a block of width `n`, aligned to the right, and has no
  effect on other values.

- `center n "leftborder" "rightborder"`:  Renders a textual
  value in a block of width `n`, aligned to the center, and has
  no effect on other values.

