There is nothing inherently tying Yesod to these languages, or the other way around: This chapter will address these template languages on their own, while the remainder of the book will use them to enhance Yesod application development. There are four main languages at play: Hamlet and Cassius are both whitespace-sensitive formats, using indentation to denote nesting.
Julius is a simple passthrough language for producing Javascript; the only added feature is variable interpolation. We mentioned in the introduction that types help protect us from XSS attacks. It might look like this:. What should happen to name , and what should its datatype be? A naive approach would be to use a Text value, and insert it verbatim. But that would give us quite a problem when name is equal to something like:.
An equally naive approach is to simply entity-encode every piece of text that gets embedded. What happens when you have some preexisting HTML generated from another process? For example, on the Yesod website, all Haskell code snippets are run through a colorizing function that wraps up words in appropriate span tags. If we entity escaped everything, code snippets would be completely unreadable!
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Instead, we have an Html datatype. In order to generate an Html value, we have two options for APIs: For the code snippet example, we would use the preEscapedToMarkup function. When you use variable interpolation in Hamlet the HTML Shakespeare language , it automatically applies a toHtml call to the value inside. So if you interpolate a String , it will be entity-escaped. But if you provide an Html value, it will appear unmodified. Possibly the most unique feature in Yesod is type-safe URLs, and the ability to use them conveniently is provided directly by Shakespeare.
Usage is nearly identical to variable interpolation; we just use the at-sign instead of the hash. Suppose we have an application with two routes: I can think of three different ways of constructing the URL:. There are problems with each approach: Just wait till you move from your development to staging and finally production server. But more importantly, there is one huge problem with all approaches: Not to mention that typos can wreak havoc as well.
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The goal of type-safe URLs is to let the compiler check things for us as much as possible. But at the end of the day, HTML is made up of text, not data types, so we need some way to convert these values to text. We call this a URL rendering function, and a simple one is:. OK, we have our render function, and we have type-safe URLs embedded in the templates. How does this fit together exactly?
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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:. 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:.
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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.
Shakespearean Templates
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? 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.
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- The Complete Works of William Shakespeare by William Shakespeare.
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. 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:.
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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:.
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