Software Simply

IntroductionThe previous twoposts in my Less Traveled Monad Tutorial series have not had much in the way of directly practical content. In other words, if you only read those posts and nothing else about monads, you probably wouldn't be able to use monads in real code. This was intentional because I felt that the practical stuff (like do notation) had adequate treatment in other resources. In this post I'm still not going to talk about the details of do notation--you should definitely read about that elsewhere--but I am going to talk about some of the most common things I have seen beginners struggle with and give you cookbook-style patterns that you can use to solve these issues.Problem: Getting at the pure value inside the monadThis is perhaps the most common problem for Haskell newcomers. It usually manifests itself as something like this: main = do lineList <- lines $ readFile "myfile.txt" -- ... do something with lineList here That code generates …

Haskell's package system, henceforth just "Cabal" for simplicity, has gotten some harsh press in the tech world recently. I want to emphasize a few points that I think are important to keep in mind in the discussion.First, this is a hard problem. There's a reason the term "DLL hell" existed long before Cabal. I can't think of any package management system I've used that didn't generate quite a bit of frustration at some point.Second, the Haskell ecosystem is also moving very quickly. There's the ongoing iteratees/conduits/pipes debate of how to do IO in an efficient and scalable way. Lenses have recently seen major advances in the state of the art. There is tons of web framework activity. I could go on and on. So while Hackage may not be the largest database of reusable code, the larger ones like CPAN that have been around for a long time are probably not moving as fast (in terms of advances in core libraries).Third, I think Haskell …

In part 1 of this tutorial we talked about types and kinds. Knowledge of kinds will help to orient yourself in today's discussion of monads. What is a monad? When you type "monad" into Hayoo the first result takes you to the documentation for the type class Monad. If you don't already have a basic familiarity with type classes, you can think of a type class as roughly equivalent to a Java interface. A type class defines a set of functions involving a certain data type. When a data type defines all the functions required by the type class, we say that it is an instance of that type class. When a type Foo is an instance of the Monad type class, you'll commonly hear people say "Foo is a monad". Here is a version of the Monad type class.class Monad m where return :: a -> m a (=<<) :: (a -> m b) -> m a -> m b(Note: If you're the untrusting type and looked up the real definition to verify that mine is accurate, you'll find th…

This is part 1 of a monad tutorial (but as we will see, it's more than your average monad tutorial). If you already have a strong grasp of types, kinds, monads, and monad transformers, and type signatures like newtype RST r s m a = RST { runRST :: r -> s -> m (a, s) } don't make your eyes glaze over, then reading this won't change your life. If you don't, then maybe it will.More seriously, when I was learning Haskell I got the impression that some topics were "more advanced" and should wait until later. Now, a few years in, I feel that understanding some of these topics earlier would have significantly sped up the learning process for me. If there are other people out there whose brains work somewhat like mine, then maybe they will be able to benefit from this tutorial. I can't say that everything I say here will be new, but I haven't seen these concepts organized in this way before.This tutorial is not for absolute beginners. It assumes a basic…

The Haskell programming language is widely considered to have a fairly steep learning curve--at least compared with mainstream languages.  In my experience with Haskell and specifically helping newcomers I've noticed a few common issues that seem to come up again and again.  Some of these issues might be more avoidable if the Haskell community did a better job communicating them.  Four points I have noticed are:
Read Haddock API docsPay attention to type class instancesLearn about kindsLearn monad transformers Read Haddock API Docs I mention this point at the risk of stating the obvious.  If you are going to become a proficient Haskell programmer, it's absolutely essential that you get used to reading the API docs for the packages you use.  I often hear newcomers ask for tutorials demonstrating how to use packages.  Our community would definitely be better off with tutorials for every package, but it would also be better if newcomers would pay more attention to the API d…

In the recent release of Heist 0.5.1, there was performance bug in my implementation of head merging. I turned off the head merging, and then fixed the bug but did not reenable head merging. If you want to turn it back on, you need to bind the html tag to the htmlImpl function from the Text.Templating.Heist.Splices.Html module. I think I'll continue to leave it off by default because it does have some potential to impact performance.

While analyzing the problem, I realized that it involves fairly obscure details of Heist internals that have not been mentioned in any of our tutorial materials. And if I, the original author of Heist fell prey to this problem, it's probably safe to assume that eventually someone else will too. So let's take a look at some details of Heist's internal behavior and the implications for splice developers.

As I alluded to in Heist in 60 Seconds, you can think of a splice as follows:

type Splice = Node -> m [Node]

The actual implementati…

In the last post I discussed how logic and control flow can (and I argue should) be done in splices where types, higher order functions, and the full power of Haskell can be applied to the problem. In this post I want to take some time to think about the raw materials that are available in templates.

At the lowest level, we have just Text (...which is certainly /= Nothing). The templates are read in as Text and parsed into HTML DOM structures: elements, children, and attributes. Heist's abstractions for these entities are roughly analogous to the lambda calculus. The lambda calculus primitives are function definition and function application. HTML structures don't have any kind of representation for functions, so Heist uses lists of DOM tree nodes as the fundamental unit. Two different ways of representing them are splices and templates. The following table summarizes Heist's HTML abstraction tools.

DefinitionApplication
/EvaluationSplices<bind>use the tagTempla…

One of the most frequent questions I get about Heist is how to do loops or control flow in Heist templates. I usually respond that we've tried to avoid introducing these constructs into templates to encourage a better separation between view and model/controller. In this post I want to discuss the issue in more detail so that my thoughts are stored and I can refer people here in the future.

Since Haskell is pure and has a strong static type system, the first thing to think about when discussing this question is what types will be involved. Since templates are processed at runtime, they do not have access to Haskell's type system. This puts us square in the middle of byte-land. No generalized looping over things like the list of registered users or recent posts. No type-safe evaluation of arbitrary expressions. It is my contention that you're better off solving these problems with a language designed for that purpose...i.e. in Haskell splices, and not in HTML templat…

A very common question when using Heist is how to specify views in a splice. There are three different approaches one can take:

1. Specify a splice's view in the Haskell code implementing the splice.
2. Get a splice's view from a template.
3. Pass the desired view to the splice as the spliced tag's child nodes.

Aproach #1 tends to be ugly and violates the principle of separating logic and view.  There are always exceptions to every rule, so we've done a little bit to ease the implementation of #1 by allowing xmlhtml DOM structures to be built with blaze-html syntax.  However, I believe that one of the other approaches will usually be preferable.

Approach #2 seems kind of awkward.  It would result in a whole bunch of small templates on disk that define little pieces of web pages.  An architecture like this might be desireable in some situations as a way to eliminate repetition (and it certainly fits into the functional programming mentality of lots of small functions),…

A template system is a bridge between static data (templates) and dynamic data.

Heist is a template system bridging HTML templates and Haskell code.

A splice is code that Heist binds to an HTML tag.

Every time the tag appears in a template, Heist runs the splice bound to that tag and passes the tag (including its attributes and its children) as input to the splice.

A splice's output is a list of tags that get substituted into the template in place of the original input tag.

Splices can be thought of as functions that can be called from templates to get dynamic data.

Splices can pass this dynamic data back to templates by (temporarily) binding new splices.

<bind> is a tag you can use in your templates to create new splices on the fly.

<apply> is a tag that lets you insert one template into another.


A more in-depth tutorial is here.

Update: Heist has switched from using hexpat to using xmlhtml as of version 0.5. You can still use blaze syntax as described in this post, but you have to import Text.Blaze.Renderer.XmlHtml instead.

Users of Heist, my Haskell templating system, got a very nice surprise for Christmas this year when Jasper Van der Jeugt uploaded a new package blaze-html-hexpat to Hackage. If you're not familiar with Heist, you should probably check out the tutorial before reading further. Heist uses the Haskell XML library hexpat for XML template parsing and internal representation. Users are encouraged to put as much markup as possible into templates and keep Haskell-generated markup to a minimum. However, it is impractical to do this 100% of the time.

When the situation arises where you do want to generate bits of markup in code, you have to generate hexpat data structures.  Hexpat does provide some convenience functions for this purpose (see the Tree and NodeClass modules), but most would ag…

I've hade some requests to explain some of the less common functions used in my log analysis screencast. I think the most straightforward approach is to examine each of the lines in a literate Haskell style. This is going to be a long-winded description of exactly what's going on. If you understood everything in the screencast, this post will probably bore you. But if you found yourself wondering what the heck was going on, this post might help.
> :m + Data.List Data.Function > contents <- readFile "user.log" > let l = lines contents > let t = map words l > mapM print $ take 2 t These four lines are pretty straightforward. ":m +" is GHCi syntax that is similar to an import. readFile :: FilePath -> IO String reads the contents of a file into a string. The lines function splits the string on newlines and creates a list of strings representing each line in the file. We map the words function over each of these lines to split the li…

Yesterday I released version 0.2 of the Heist XML templating library. This release makes some significant API changes, so it may break existing applications. But if you're only using Heist for basic templating, then you probably won't have to change anything. However, http://snapframework.com uses Heist extensively; and it did not require any code changes to upgrade. Here is a summary of what changed in 0.2: String substitution in attributesSupport for DOCTYPE in templatesNew implementation for TemplateMonadWindows supportTypeable instanceDocumentation improvementsBug fixesThe biggest new feature in 0.2 is attribute string substitution. Because Heist's templating mechanism uses XML tags, it didn't automatically work for string substitution inside tag attributes. You don't have to do very much web development before you find yourself wanting this. Now you can get substitution in attributes using the syntax "$(name)". The Heist Tutorial describes th…

The other day I wanted to analyze some of my website log files to get a better idea of how many active users I have. I've been meaning to do this for quite some time, but have kept putting it off. I decided to see what I could accomplish by just doing some experimenting in GHCI. It was so easy and convenient that I decided to do a screencast demonstrating what I did and how easy it was. The conciseness of Haskell combined with the instant feedback of an interpreter make a very powerful combination. Here's the screencast on vimeo.

In developing the HAppS example that I have been posting here, I came upon a problem that gave me new insight to Haskell. I don't think it was a particularly deep insight, but it is significant to me as someone new to the language, and new to type inference.Consider a user authentication function that retrieves a map from the reader monad, looks up a username, and compares the password retrieved with a passed in parameter. I first implemented this function something like this: data User = User { username :: String, password :: String } authUser name pass = do u <- liftM (M.lookup name) ask liftM2 (==) (liftM password u) (return pass) In the process of getting there, I stumbled around rearranging and inserting various liftM calls until I finally got it to work. Many of the reasons behind the type errors still seemed like voodoo to me. And my haphazard approach to fixing them is evident looking at the code. The problem with th…

An astute reader pointed out that there is a transactional integrity problem with the HAppS application built over the last 4 posts. The function checkAndAdd in Finished HAppS Application contains a call to "query $ IsUser" as well as a call to "update $ AddUser". This violates that ACID guarantee that was desired from the checkAndAdd function. If two people simultaneously try to create the same username, it's possible that both of them could get past the "query" and "if exists" statements before either of the "update AddUser" statements are executed. In this case, both of the AddUser updates would succeed and both users would think their account was created. But if they had the same username, then first one would be overwritten by the second one. The second user wouldn't notice a problem, but the first user would not be able to log in to the newly created account because his password would probably be different from the …

Update: Due to popular demand, I put the plain haskell code for this app on hpaste.org. Here are links for Session.hs, Main.hs, and login.html. The past three posts have laid the groundwork for a full HAppS web app. We have built the infrastructure for an authentication system that can store usernames and passwords as well as the session IDs for active sessions. Here we will tie everything together to make a functional web app. It should all compile with HAppS 0.9.2. Let's get the imports out of the way. > {-# OPTIONS -fglasgow-exts #-} > {-# LANGUAGE TemplateHaskell , FlexibleInstances, > UndecidableInstances, OverlappingInstances, > MultiParamTypeClasses, GeneralizedNewtypeDeriving #-} > > module Main where > > import Control.Concurrent > import Control.Monad > import HAppS.Server > import HAppS.State > > import Session --The session and state code already developed In the first post, we created code to se…

Update: A demo of the finished application is now available. See this post for more information.In the last post, I outlined the requirements for making a data type an instance of Component. This is great, but not very useful without a mechanism for accessing the state data.HAppS persists its state by storing functions that operate on the state. This requires a way to serialize the functions. HAppS does this for you with the TemplateHaskell function mkMethods. So how does this affect you? Your functions that manipulate state must be either Update or Query functions. Update functions use the State monad, and Query functions use the Reader monad.First we'll set up some convenience functions that will be used to construct the actual Query and Update functions. > askUsers :: MonadReader State m => m (M.Map String User) > askUsers = return . users =<< ask > > askSessions::MonadReader State m => m (Sessions SessionData) > askSessions = return . session…

Update: A demo of the finished application is now available. See this post for more information.This post is written in literate haskell, so it should compile as-is in a .lhs file.In my last article on HAppS, I gave a brief introduction to working with the HAppS web server to serve a basic user login/registration page and handle the form submission. In this article we are going to develop the framework for basic session management. The example file AllIn.hs in the HAppS source tree is very similar to this. I have made a few changes to demonstrate some different cases that one might encounter. So without further ado, we'll start with our standard import statements. > {-# OPTIONS -fglasgow-exts #-} > {-# LANGUAGE TemplateHaskell , FlexibleInstances, > UndecidableInstances, OverlappingInstances, > MultiParamTypeClasses, GeneralizedNewtypeDeriving #-} > > module Session where > > import qualified Data.Map as M > import Contro…

Update: A demo of the finished application is now available. See this post for more information.This post is the first in a series outlining the process I am taking to get a basic stateful web application working with the Haskell web framework HAppS. There is very little documentation available for versions of HAppS after 0.8.8. While this is not documentation, it should aid in getting an idea of how HAppS manages state. The code in these posts should work with HAppS 0.9.2.For this application, we'll focus on the basic capabilities needed for user creation, authentication, and session management. The first thing we need is a form for logging in or creating a new user, and two URLs to handle the submission of these forms. Well use /login for the form, and we'll have the login form POST to the same place. The registration form will POST to /newuser. All other pages will return an error. Here is the basic HAppS code to do that: impl = [ dir "login" [methodSP GE…