OK, we have our render function, and we have type-safe URLs embedded in the templates. How does this fit together exactly? Instead of generating an Html or Css or Javascript value directly, Shakespearean templates actually produce a function, which takes this render function and produces HTML. Supposing we had a template:.
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All Shakespearean languages share the same interpolation syntax, and are able to utilize type-safe URLs. Hamlet is the most sophisticated of the languages. Not only does it provide syntax for generating HTML, it also allows for basic control structures: Obviously tags will play an important part of any HTML template language. However, instead of using closing tags to denote nesting, we use indentation.
So something like this in HTML:.
In general, we find this to be easier to follow than HTML once you get accustomed to it. The only tricky part comes with dealing with whitespace before and after tags. We want to make sure that whitespace is preserved after the word "Paragraph" and before the word "end".
To do so, we use two simple escape characters:. If the first non-space character in a line is a backslash, the backslash is ignored. Hamlet does not escape entities within its content. This is done on purpose to allow existing HTML to be more easily copied in. So the example above could also be written as:. Notice that the first tag will be automatically closed by Hamlet, while the inner "i" tag will not.
You are free to use whichever approach you want, there is no penalty for either choice. Be aware, however, that the only time you use closing tags in Hamlet is for such inline tags; normal tags are not closed. Another outcome of this is that any tags after the first tag do not have special treatment for IDs and classes. For example, the Hamlet snippet:. Notice how the p tag is automatically closed, and its attributes get special treatment, whereas the i tag is treated as plain text.
How do we pass in variables?
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The hash followed by a pair of braces denotes variable interpolation. In the case above, the title variable from the scope in which the template was called will be used. Let me state that again: There is no need to specifically pass variables in. You can apply functions within an interpolation. You can use string and numeric literals in an interpolation. You can use qualified modules. Both parentheses and the dollar sign can be used to group statements together. And at the end, the toHtml function is applied to the result, meaning any instance of ToHtml can be interpolated.
Take, for instance, the following code. What about our much-touted type-safe URLs? They are almost identical to variable interpolation in every way, except they start with an at-sign instead. The next code sample demonstrates both of these. Additionally, there is a variant of URL interpolation which allows you to embed query string parameters. This can be useful, for example, for creating paginated responses.
The value you provide must be a two-tuple with the first value being a type-safe URL and the second being a list of query string parameter pairs. See the next code snippet for an example. In that last example, we put an href attribute on the "a" tag. The equals sign and value for an attribute are optional, just like in HTML.
There are two convenience attributes: While quotes around the attribute value are optional, they are required if you want to embed spaces. You can add an attribute optionally by using colons. This also works for class names, e. The interpolated variable must be a tuple, or list of tuples, of Text or String.
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The goal of Hamlet is to make the logic as minimalistic as possible, pushing the heavy lifting into Haskell. Similarly, we have a special construct for dealing with Maybe values. This could technically be dealt with using if , isJust and fromJust , but this is more convenient and avoids partial functions. In addition to simple identifiers, you can use a few other, more complicated values on the left hand side, such as constructors and tuples.
The right-hand-side follows the same rules as interpolations, allow variables, function application, and so on. Pattern matching is one of the great strengths of Haskell. Sum types let you cleanly model many real-world types, and case statements let you safely match, letting the compiler warn you if you missed a case. Hamlet gives you the same power. Rounding out our statements, we have with. Last bit of syntactic sugar: Lucius is one of two CSS templating languages in the Shakespeare family. It is intended to be a superset of CSS, leveraging the existing syntax while adding in a few more features.
Starting with the second point: Imagine if you had a dozen or so of these. Not the worst thing in the world, but a bit of an annoyance. Lucius helps you out here:. Having Lucius variables allows you to avoid repeating yourself. A simple example would be to define a commonly used color:. Mixins are a relatively new addition to Lucius.
The following example demonstrates how we could use a mixin to deal with vendor prefixes. Cassius is a whitespace-sensitive alternative to Lucius. As mentioned in the synopsis, it uses the same processing engine as Lucius, but preprocesses all input to insert braces to enclose subblocks and semicolons to terminate lines. This means you can leverage all features of Lucius when writing Cassius. As a simple example:.
Julius is the simplest of the languages discussed here. The question of course arises at some point: There are three different ways to call out to Shakespeare from your Haskell code:. Quasiquotes allow you to embed arbitrary content within your Haskell, and for it to be converted into Haskell code at compile time. Both of the above modes require a full recompile to see any changes. In reload mode, your template is kept in a separate file and referenced via Template Haskell.
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But at runtime, the external file is reparsed from scratch each time. One of the first two approaches should be used in production. They both embed the entirety of the template in the final executable, simplifying deployment and increasing performance. The advantage of the quasiquoter is the simplicity: For short templates, this can be a very good fit. However, in general, the external file approach is recommended because:. You can easily switch between external file and debug mode with some simple CPP macros, meaning you can keep rapid development and still achieve high performance in production.
Since these are special QuasiQuoters and Template Haskell functions, you need to be sure to enable the appropriate language extensions and use correct syntax. You can see a simple example of each in the following code snippets. Hamlet HtmlUrl, hamlet import Data. Text Text import Text. Text Text import qualified Data. There are currently three other values we can generate using Hamlet: That last one will be covered in more detail in the widgets chapter.
There are a few changes:. We use a different set of functions, prefixed with an "s". So the quasiquoter is shamlet and the external file function is shamletFile. How we pronounce those is still up for debate. Embedding the caret-interpolator no longer allows arbitrary HtmlUrl values. The rule is that the embedded value must have the same type as the template itself, so in this case it must be Html. That means that for shamlet , embedding can be completely replaced with normal variable interpolation with a hash.
Dealing with internationalization i18n in Hamlet is a bit complicated. Hamlet supports i18n via a message datatype, very similar in concept and implementation to a type-safe URL. We could represent those messages with a datatype. Next, we would want to be able to convert that into something human-readable, so we define some render functions:. Now we want to interpolate those Msg values directly in the template.
For that, we use underscore interpolation. This kind of a template now needs some way to turn those values into HTML. Information from its description page there is shown below. Commons is a freely licensed media file repository.
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The titular tempest wrecks the ship of King Alonso. As revealed in Act I, Scene 2, the tempest was conjured by Prospero old man, right , who sent the spirits seen in the upper left to create the storm. His daughter Miranda clings to him, as she does in Scene 2, begging for the lives of those on the vessel - but Prospero assures her used his magic to prevent anyone from dying, though he will lead people to believe that others have in the events to come in the play.
His brother, Antonio, who usurped his place as the Duke of Milan, is on the vessel, and it is against him he seeks revenge Boydell au sein de la Boydell Shakespeare Gallery. Mais Prospero la rassure: This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required.
Licensing for more information. This is a retouched picture , which means that it has been digitally altered from its original version. Heavy cleanup to rather damaged left side, light cleanup elsewhere, levels adjustment, crop. Ariel and other spirits, causing the tempest. The rest of the main characters, except Caliban , and the mariners.
Been learning the LoC aren't always the best guides to colour of things. Slightly more yellow as I noticed it was coming out a little pinkish on the school monitor. The following pages on the English Wikipedia link to this file pages on other projects are not listed:. The following other wikis use this file: Retrieved from " https: Featured pictures Wikipedia Picture of the day files.
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