Yesod (Web framework)

Server interface

Yesod uses a Web application interface API, abbrev. WAI, to isolate servlets, aka web apps., from servers, with handlers for the server protocols CGI, FastCGI, SCGI, Warp, Launch (open as local URL to the default browser, closing the server when the window is closed),

The foundation type

See ref. Yesod requires a data type that instantiates the controller classes. This is called the foundation type. In the example below, it is named "MyApp".

The REST model identifies a web resource with a web path. Here REST resources are given names with an R suffix (like "HomeR") and are listed in a parseRoutes site map description template. From this list, route names and dispatch handler names are derived.

Yesod makes use of Template Haskell metaprogramming to generate code from templates at compile time, assuring that the names in the templates match and everything typechecks (e.g. web resource names and handler names).

By inserting a mkYesod call, this will call T.H. primitives to generate the code corresponding to the route type members, and the instances of the dispatch controller classes as to dispatch GET calls to route HomeR to a routine named composing them both as "getHomeR", expecting an existing handler that matches the name.

Skeleton app. extracted from next section, "Hello world" example:

Resources, routes and HTTP method handlers

See ref. Yesod follows the REpresentational State Transfer model of access to web documents, identifying docs. and directories as resources with a Route constructor, named with an uppercase R suffix (for example, HomeR).

The routes table

The parseRoutes template should list the resources specifying route pieces, resource name and dispatch methods to be accepted.

Applying the previous template generates the following route constructors:

Handlers

For every HTTP method a handler function must be created to match the dispatch names generated by mkYesod from the parseRoutes template, by prefixing the method name (or the prefix handler if no method stated) to the resource, as described:

Request data, Parameters, Cookies, Languages and other Header info

See ref.

Authentication and authorization

See ref. Authentication plugins: OpenId, BrowserId, Email, GoogleEmail, HashDB, RpxNow.

Redirection after authentication.

Sessions

See ref. Session back-ends: ClientSession.

>> To avoid undue bandwidth overhead, production sites can serve their static content from a separate domain name to avoid the overhead of transmitting the session cookie for each request

Subsites

See ref.

>> A subsite is a collection of routes and their handlers that can be easily inserted into a master site.

Built-in subsites: Static, Auth

Subsite Static

For every file in the "static" folder, a symbol with type (Route Static) is generated for reference, by means of a compile time splice call in the scaffold module StaticFiles.hs, that replaces non-identifier characters "/-." with underscores:

After adding static files, regenerate (::Route Static) symbols at the next recompilation, just updating the StaticFiles.hs date:

View

The Handler monad returns content in one or more of several formats as components of types that implement the HasReps class {RepHtml, RepJson, RepXml, RepPlain, the dual RepHtmlJson, a pair or list of pairs [(ContentType, Content)], ..}. Json examples:

The HasReps default implementation of chooseRep chooses the document representation to be returned according to the preferred content-type list of the client accept header.

Widgets are HTML DOM code snippets made by specific commands (e.g. setTitle) or from templates of structure (html) / behaviour (javascript) / style (css), whose types instantiate the classes ToWidget, ToWidgetHead or ToWidgetBody.

A Widget monad, based on a Writer one and argument to defaultLayout, facilitate to piece the widgets together.

Template interpolation - Shakespearean templates

See ref. These are content view templates that follow a common substitution pattern of code expressions within curly brackets with different character prefix to refer to

template expressions with ^{...}

other templates of the same type as ^{template params},

route expressions with @{...}

safe (typed) urls as @{HomeR},

message expressions with _{...}

i18n message rendering as _{MsgMessage params}

other Haskell expressions with #{...}

haskell expression rendering as #{haskell_expression} which type must be convertible

in case of hamlet html templates, the expression type must be an instance of Text.Blaze.ToMarkup

in case of css templates, the expression type must be an instance of Text.Cassius.ToCss

in case of javascript templates, the expression type must be an instance of Text.Julius.ToJavascript

in case of i18n localizable msg expressions _{msgExpr} in hamlet templates, the expression type must be an instance of Text.Shakespeare.I18N.ToMessage

in case of plain text templates, the expression type must be an instance of Text.Shakespeare.Text.ToText

Using non-English text in expressions requires use of the Unicode-aware type Text, since GHC's show for the type String renders non-ASCII characters as escaped numerical codes.

external file templates: Template content can be loaded from external files using compile time splice calls as $(expr).

reload mode for external files: See doc.

Localizable (i18n) messages

See ref. For every supported language ISO name there should be a file in the messages subfolder as <iso-language>.msg with entries like

ArticleUnexistant param@Int64: unexistant article #{param}

For each entry in en.msg a message constructor is generated, prefixing the message name by "Msg", so the example msg. can be referred as

-- in code myMsg = MsgArticleUnexistant myArticleId -- in templates _{MsgArticleUnexistant myArticleId}

HTML-like templates

the hamlet quasiquoter (a parser to compile-time Template Haskell code) specified in the T.H. Oxford brackets syntax [qq| ... |] introduces an indentation based structured html template with '$' prefixed lines of logic statements (See doc.). Automatic closing tags are generated only for the tag at line start position.

the whamlet quasiquoter returns a Widget expression. (saves toWidget before [hamlet|..|]).

JavaScript templates

the julius quasiquoter: introduces a javascript template. Javascript variants CoffeeScript and Roy-language have also specific quasiquoters.

CSS-like templates

the cassius quasiquoter: introduces a css template with indentation based structuring.

the lucius quasiquoter: introduces a css template with standard syntax plus shakespeare-template style substitutions.

Plain text templates

the lt (lazy text, same as stext) and st (strict text) quasiquoters: they introduce Text templates, for e-mail or text/plain http content type.

[lt| Mr./Mrs. #{fullName} ... |]

Specific views

Search engines XML Sitemaps, where sitemap returns an XML Sitemap as http response, with the routes we want the search engines to crawl, and attributes to instruct the crawler, from a provided list of SitemapUrl records.

Navigation Breadcrumbs. You have to provide a YesodBreadcrumbs instance for the site where the generator function breadcrumb should return a title and parent route for each one. Then, the query function breadcrumbs will return the present route title and ancestors' (route, title) pairs.

Web feed views (RSS / Atom). You have handlers that return RepRss, RepAtom, or dual RepAtomRss content (to be selected on accept headers' preferred content-type list) from a given Feed structure.

Using in-memory mutable data (in the foundation datatype)

E.g. a visitor count. See ref.

Persistent

persistent is the name of the database access layer with templates for generating types for entities and keys as well as schema initialization.

There is first class support for PostgreSQL, SQLite, MongoDB, CouchDB and MySQL, with experimental support for Redis.

automatic table creation, schema update and table migration

Modifications of the entities template produces an schema update with automatic table creation, and migration for the DBMS's that support "ALTER TABLE" SQL commands in a migrateAll procedure, generated from the template content. See "Migrations" in ref. to look for migration aware DBMS.

Esqueleto: is a haskell combinators layer to generate correct relational queries to persistent.

Example for persistent rawSQL and Esqueleto queries.

Default column values for added columns in automatic migrations.

Forms

See ref. There are Applicative, Monadic and Input (non rendering, input only) kinds of forms.

Field definitions have a fieldParse component and a fieldView one.

the function runForm{Post|Get} runs the field parsers against the form field inputs and generates a (FormResult, Widget) pair from the views with param. values as defaults,

while generateForm{Post|Get} ignores the web inputs and generates a blank form widget.

The magic is in the Applicative and Functor instances of the data type FormResult, where (<*>) collects the error messages for the case of FormFailure [textErrMsg] result values

Monadic forms permit free form layout and better treatment of hiddenField members.

A sample of an Applicative form:

See renderMessage signature.

E-mail

The following packages are part of the yesod-platform:

email-validate: Validating an email address.

mime-mail: Compose and send MIME email messages.

Facebook

Useful glue functions between the fb library and Yesod.

Getting started

Create a sandboxed cabal-dev repository for yesod and yesod projects. Yesod 1.2 release notes.

Scaffolding

The console command yesod init after asking for details, generates a starting scaffold application in a subfolder with the project name. Then cd to the project folder.

Create a link to the parent dir. cabal-dev folder (if you want to use the same library repository) and build the project

Configuration environments

They are predefined as {Development, Testing, Staging, Production} and each one heads a chapter in the YAML formatted configuration files ("config/" based settings.yml and <yourDBMS>.yml), with configuration attributes for the purpose stated by the environment name.

For each one, several attributes can be specified, e.g.: base url as approot, port, and other configurations, as well as different database names, its connection parameters, and other db parameters adjustable for the purposes of the specific environment.

You have to specify the name of the environment as argument to your project task. Example:

Developing

The console command yesod --dev devel --port 3000 (the --dev flag is to look for the cabal-dev library repository) compiles the project in the current folder and starts it as a web server, but also listens for file modifications in the project directory tree, and recompiles and restarts it every time you save a yesod component, whether haskell code or template file.

Adding static files requires to unix touch the Settings/StaticFiles.hs module to generate the correspondent route symbols as explained.

Logging for debug

See ref. The yesod scaffold uses "wai-extra" Network.Wai.Middleware.RequestLogger for request logging, although there are alternatives.

The package "monad-logger" brings T.H. generated logging functions with automatic inclusion of line location, which can be used in the monads Handler, Widget, PersistQuery, PersistStore, .. which are instances of MonadLogger.

The following "monad-logger" calls are also available from "yesod-core":

You can set per case log default by overriding the shouldLog method of the Yesod class instance for the site.

Deploying

See ref.

Keter: A web app server monitor and reverse proxy server

See refs.

Keter is a process as a service that handles deployment and restart of web app servers, and, per web app, database creation for PostgreSQL.

The console command yesod --dev keter packs the web app. as a keter bundle for uploading to a keter folder named "incoming".

Keter monitors the "incoming" folder and unpacks the app. to a temporary one, then assigns the web app a port to listen to, and starts it.

Initially it worked with Nginx as reverse proxy (keter version 0.1*), adding virtual server entries to its configuration and making Nginx reload it, but now Keter itself provides its own reverse proxy functionality, removing Nginx dependency and acting as the main web server.

Old documentation (Nginx based).

Integration with JavaScript generated from functional languages

See ref.

from true GHC Haskell with hacked compilers having JavaScript backends

Haste and GhcJs offer alternatives that generate Javascript from the STG output phase of GHC admitting haskell code compilable with GHC. While GhcJs is more feature complete (concurrency, etc.), it requires and generates much more code than Haste.

Haste sample. Other examples at the ref.

Equivalent with reactive data-flow style.

Compiling:

Yesod widgets code:

Elm - reactive composition of web user interfaces

Elm is a novel functional reactive programming language (no callbacks) with Haskell like syntax (type annotations included) that compiles to Javascript, but with strict evaluation (top down evaluation, no where clauses), with simplified syntax (single pattern definitions, no guards), propagating events and changes from event sources through the dependency graph.

Elm role is not for adding JavaScript behaviour to existing elements, but to build up a variable html structure that recomposes itself in reaction to event streams.

Elm is more oriented to graphics than to text flows. Since everything can be scaled, moved and rotated, it needs precise knowledge of dimensions, so it doesn't use html lists, paragraphs and tables, whose positioning rely on the browser. With Elm you put text or graphics in rectangular Elements that can be stacked from lists (with Graphics.Element.flow direction elements) along horizontal, vertical or layer dimensions.

It unifies events and behaviours (varying values) in an effect named Signal handled as a Haskell applicative functor. You define value transformation functions and then you lift these functions to apply them to the Signal effect.

Elm calculator as example:

Instead of mangling the DOM tree nodes, Elm builds Html structure by composing Elements (html block elements) (with flow, container, layers or empty from Graphics.Element), from

appendable Text (html inline elements) (library Text),

stackable graphics (type named Form) as a collage (html canvas) (library Graphics.Collage),

composable input elements (html form elements) (library Graphics.Input), image elements (library Graphics.Element) and container elements

Conditional structures may switch to Element.empty.

Styling. Elm needs to know precisely the width and height of elements, so styling is done through Elm styling functions instead of leaving it to external CSS. It lacks margins and paddings for now. You make element (html block) styles as a composition of (Element -> Element) styling functions, and text (html inline) styles in a similar way (Text -> Text).

Instead of web forms, you wrap the status signals of the input elements in a Request signal as input to Elm's Ajax Http.send

The main function effects-sequencing pattern is Applicative (there are no do blocks). You can add state effects through past-dependent folds (Signal.foldp, Signal.count) or state Automatons (an arrow instance) There is a third-party library repository

Elm requires

an empty div html element (with an ID), where to hook the varying content result,

and a client JavaScript snippet to start the compiled script as shown in Elm#Parameterizing an Elm script

Styling is not as good as CSS, url encoding requires a third party library, but the language works fairly well. It seems more geared to build game UI's but with some extra work can do ajax web forms as well.

Disadvantages:

No Html lists, so no list items, you have to paint the bullets yourself.

Styling is rudimentary. No margins, no paddings, no paragraphs, no CSS.

interfacing with JavaScript functions is not allowed (blamed impure). Only values or signal streams are admitted.

Advantages:

Html built by composition in reaction to Signals.

Good to make an animated collage, as this is the name of its graphics library.

Using Elm with Yesod in a remote echo example

XmlHttpRequest (in Elm's Http.send) requires server side authorization headers "Access-Control-Allow-Origin: *" for Http GET and extra "Access-Control-Allow-Methods: GET, POST" for Http POST. Url encoding of requests is implemented in a third-party library named elm_CodecURI.

Install Elm and compile:

Yesod handler to return echo of "status" parameter.

Templates for inclusion of Elm functionality