Copyright | Copyright (C) 2009-2019 John MacFarlane |
---|---|
License | BSD3 |
Maintainer | John MacFarlane <jgm@berkeley.edu> |
Stability | alpha |
Portability | portable |
Safe Haskell | None |
Language | Haskell2010 |
This is the text templating system used by pandoc. Its basic function is
to fill holes in a template in a Context
that provides values for
variables. Control structures are provided to test that 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 also supported.
Templates may be rendered to lazy or strict Text
, String
, or a
doclayout Doc
. (Using a Doc
allows rendered documents to wrap
flexibly on breaking spaces.) A Context
can be constructed manually or
an aeson Value
may be used.
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
import Data.Text (Text) import qualified Data.Text.IO as T import Data.Aeson import Text.DocTemplates 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$$sep$\n$endfor$" main :: IO () main = do res <- compileTemplate "mytemplate.txt" template case res of Left e -> error e Right t -> T.putStrLn $ renderTemplate t $ object ["employee" .= [ Employee "John" "Doe" Nothing , Employee "Omar" "Smith" (Just 30000) , Employee "Sara" "Chen" (Just 60000) ] ]
Delimiters
To mark variables and control structures in the template, either $
…$
or ${
…}
may be used as delimiters. The styles may also be mixed in
the same template, but the opening and closing delimiter must match in
each case. The opening delimiter may be followed by one or more spaces
or tabs, which will be ignored. The closing delimiter may be followed by
one or more spaces or tabs, which will be ignored.
To include a literal $
in the document, use $$
.
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 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.
- If the value is a JSON boolean, it will be rendered as
true
if true, and as the empty string if false.
The value of a variable that occurs by itself on a line will be indented to the same level as the opening delimiter of the variable.
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 non-empty value, otherwise the
else
section is used (if present). (Note that even the string false
counts as a true value.) 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. If the
value of the associated variable is not an array, 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 }
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.
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
+reflow
keyword (ended with -reflow
).
${ +reflow }This long line may break if the document is rendered with a short line length.${ -reflow }
The +
keyword has no effect when rendering to Text
or String
.
Nesting
As noted above, the value of a variable that occurs by itself on a line will be indented to the same level as the opening delimiter of the variable.
In addition, any part of a template can be marked explicitly for
indented rendering, using the +nest
keyword (o start nesting at the
column where it appears) and -nest
to stop nesting.
Example:
$for(article)$ - $+nest$$article.author$, "$article.title$," in $article.book$ ($article.year$).$-nest$ $endfor$
Synopsis
- renderTemplate :: (TemplateTarget a, ToContext a b) => Template -> b -> a
- compileTemplate :: TemplateMonad m => FilePath -> Text -> m (Either String Template)
- compileTemplateFile :: FilePath -> IO (Either String Template)
- applyTemplate :: (TemplateMonad m, TemplateTarget a, ToContext a b) => FilePath -> Text -> b -> m (Either String a)
- class Monad m => TemplateMonad m where
- getPartial :: FilePath -> m Text
- class Monoid a => TemplateTarget a where
- fromText :: Text -> a
- toText :: a -> Text
- removeFinalNewline :: a -> a
- isEmpty :: a -> Bool
- indent :: Int -> a -> a
- breakingSpace :: a
- newtype Context a = Context {}
- data Val a
- class ToContext a b where
- class FromContext a b where
- data Template
Documentation
renderTemplate :: (TemplateTarget a, ToContext a b) => Template -> b -> a Source #
Render a compiled template in a "context" which provides values for the template's variables.
compileTemplate :: TemplateMonad m => FilePath -> Text -> m (Either String Template) Source #
Compile a template. The FilePath parameter is used to determine a default path and extension for partials and may be left empty if partials are not used.
compileTemplateFile :: FilePath -> IO (Either String Template) Source #
Compile a template from a file. IO errors will be raised as exceptions; template parsing errors result in Left return values.
applyTemplate :: (TemplateMonad m, TemplateTarget a, ToContext a b) => FilePath -> Text -> b -> m (Either String a) Source #
Compile a template and apply it to a context. This is
just a convenience function composing compileTemplate
and renderTemplate
. If a template will be rendered
more than once in the same process, compile it separately
for better performance.
class Monad m => TemplateMonad m where Source #
A TemplateMonad
defines a function to retrieve a partial
(from the file system, from a database, or using a default
value).
getPartial :: FilePath -> m Text Source #
Instances
TemplateMonad IO Source # | |
Defined in Text.DocTemplates.Internal | |
TemplateMonad Identity Source # | |
Defined in Text.DocTemplates.Internal |
class Monoid a => TemplateTarget a where Source #
A type to which templates can be rendered.
fromText :: Text -> a Source #
removeFinalNewline :: a -> a Source #
indent :: Int -> a -> a Source #
breakingSpace :: a Source #
Instances
TemplateTarget Text Source # | |
TemplateTarget Text Source # | |
TemplateTarget String Source # | |
(HasChars a, IsString a, Eq a) => TemplateTarget (Doc a) Source # | |
A Context
defines values for template's variables.
Instances
Functor Context Source # | |
Foldable Context Source # | |
Defined in Text.DocTemplates.Internal fold :: Monoid m => Context m -> m # foldMap :: Monoid m => (a -> m) -> Context a -> m # foldr :: (a -> b -> b) -> b -> Context a -> b # foldr' :: (a -> b -> b) -> b -> Context a -> b # foldl :: (b -> a -> b) -> b -> Context a -> b # foldl' :: (b -> a -> b) -> b -> Context a -> b # foldr1 :: (a -> a -> a) -> Context a -> a # foldl1 :: (a -> a -> a) -> Context a -> a # elem :: Eq a => a -> Context a -> Bool # maximum :: Ord a => Context a -> a # minimum :: Ord a => Context a -> a # | |
Traversable Context Source # | |
TemplateTarget a => ToContext a (Context a) Source # | |
Show a => Show (Context a) Source # | |
Semigroup (Context a) Source # | |
Monoid (Context a) Source # | |
TemplateTarget a => ToJSON (Context a) Source # | |
Defined in Text.DocTemplates.Internal | |
TemplateTarget a => FromJSON (Context a) Source # | |
A variable value.
Instances
Functor Val Source # | |
Foldable Val Source # | |
Defined in Text.DocTemplates.Internal fold :: Monoid m => Val m -> m # foldMap :: Monoid m => (a -> m) -> Val a -> m # foldr :: (a -> b -> b) -> b -> Val a -> b # foldr' :: (a -> b -> b) -> b -> Val a -> b # foldl :: (b -> a -> b) -> b -> Val a -> b # foldl' :: (b -> a -> b) -> b -> Val a -> b # foldr1 :: (a -> a -> a) -> Val a -> a # foldl1 :: (a -> a -> a) -> Val a -> a # elem :: Eq a => a -> Val a -> Bool # maximum :: Ord a => Val a -> a # | |
Traversable Val Source # | |
TemplateTarget a => FromContext a (Val a) Source # | |
TemplateTarget a => ToContext a (Val a) Source # | |
Show a => Show (Val a) Source # | |
TemplateTarget a => ToJSON (Val a) Source # | |
Defined in Text.DocTemplates.Internal | |
TemplateTarget a => FromJSON (Val a) Source # | |
class ToContext a b where Source #
Instances
TemplateTarget a => ToContext a Bool Source # | |
TemplateTarget a => ToContext a Value Source # | |
TemplateTarget a => ToContext a a Source # | |
ToContext String String Source # | |
ToContext a b => ToContext a [b] Source # | |
TemplateTarget a => ToContext a (Val a) Source # | |
TemplateTarget a => ToContext a (Context a) Source # | |
(TemplateTarget a, HasChars a) => ToContext (Doc a) a Source # | |
class FromContext a b where Source #
The FromContext
class provides functions for extracting
values from Val
and Context
.
Instances
TemplateTarget a => FromContext a a Source # | |
FromContext String String Source # | |
FromContext a b => FromContext a [b] Source # | |
TemplateTarget a => FromContext a (Val a) Source # | |
A template.