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Learning Functional Programming through Multimedia
As the title implies, The Haskell School of Expression introduces functional programming through the Haskell programming language and through the use of graphics and music. It serves as an effective introduction to both the language and the concepts behind functional programming. This text was published in 2000, but since Haskell 98 is the current standard, this is still a very relevant book.
Haskell's standardization process gives us a window into two different facets of the community: Haskell is designed to be both a stable, standardized language (called Haskell 98), and a platform for experimentation in cutting-edge programming language research. So though we have a standard from 1998, the implementations (both compilers and interpreters) are continually evolving to implement new, experimental features which may or may not make it into the next standard.
For instance, the Glasgow Haskell Compiler has implemented a meta-programming environment called Template Haskell. Haskell is also easy to extend in directions that don't change the language itself, through the use of Embedded Domain-Specific Languages (EDSLs) such as WASH for web authoring, Parsec for parsing, and Dance (more of Paul Hudak's work) for controlling humanoid robots.
Before we get too far, I should offer a disclaimer: The Haskell community is rather small, and if you scour the net, you may find conversations between myself and Paul Hudak or folks in his research group, since I use some of their software. That said, I don't work directly with Hudak or his research group.
In fact, the small size of the Haskell community is a useful feature. It is very easy to get involved, and folks are always willing to help newbies learn, since we love sharing what we know. You may even find that if you post a question about an exercise in The Haskell School of Expression , you'll get a reply from the author himself.
I consider this book to be written in a "tutorial" style. It walks the reader through the building of applications, but doesn't skimp on the concepts (indeed, the chapters are meant to alternate between "concepts" and "applications"). In some ways, the code examples make it a little difficult to jump around, since you are expected to build upon previous code. The web site provides code, however, so you can always grab that and use it to fill in the missing pieces.
For readers who wish to use this book as a tutorial, and to implement all of the examples (which is highly recommended), I suggest that you grab the Hugs interpreter and read the User's Guide while you're reading the first few chapters of The Haskell School of Expression. Hugs is very portable, free, and easy to use. It also has an interface with Emacs. Unfortunately, some of the example code has suffered from bit-rot, and certain things don't work out-of-the-box for X11-based systems. The bit-rot can be solved by using the "November 2002" version of Hugs. This is all explained on SOE's web page.
The Haskell School of Expression should be very effective for programmers who have experience in more traditional languages, and programmers with a Lisp background can probably move quickly through some of the early material. If you've never learned a functional language, I highly recommend Haskell: Since Haskell is purely functional (unlike Lisp), it will more or less prevent you from "cheating" by reverting to a non-functional style. In fact, if you've never really looked at functional programming languages, it may surprise you to learn that Haskell has no looping constructs or destructive assignment (that is, no x = x + 1). All of the tasks that you would accomplish through the use of loops are accomplished instead through recursion, or through higher-level abstractions upon recursion.
Since I was already comfortable with recursion when I started this book, it is hard for me to gauge how a reader who has never encountered recursion would find this book's explanation of the concept. The Haskell School of Expression introduces recursion early on, in section 1.4. It is used in examples throughout the book, and if you follow along with these examples, you will most certainly be using it a lot. The introduction seems natural enough to me, but I note that Hudak does not give the reader any extra insight or tricks to help them along. Not to worry, though; recursion is very natural in Haskell and the reader may not even notice that they are doing something a little tricky.
The use of multimedia was a lot of fun for me, and should quickly dispel the myth that IO is difficult in Haskell. For instance, Hudak has the reader drawing fractals by page 44, and throughout the book, the reader will be drawing shapes, playing music, and controlling animated robots.
Any book on Haskell must be appraised for its explanation of monads in general and IO specifically. Monads are a purely functional way to elegantly carry state across several computations (rather than passing state explicitly as a parameter to each function). They are a common stumbling block in learning Haskell, though in my opinion, their difficulty is over-hyped.
Since input and output cause side-effects, they are not purely functional, and don't fit nicely into a function-call and recursion structure. Haskell has therefore evolved a way to deal safely and logically with IO through the use of monads, which encapsulate mutable state. In order to perform IO in Haskell, one must use monads, but not necessarily understand them.
Some people find monads confusing; I've even heard a joke that you need a Ph.D. in computer science in order to perform IO in Haskell. This is clearly not true, and this book takes an approach which I whole-heartedly agree with. It gets the reader using monads and IO in chapter 3 without explaining them deeply until chapters 16 (IO) and 18 (monads). By the time you get there, if you have heard that monads are confusing, you might be inclined to say "how is this different from what we've been doing all along?" Over all, I was pleased with the explanation of monads, especially state monads in chapter 18, but I felt that the reader is not given enough exercises where they implement their own monads.
If you're worried that drawing shapes and playing music will not appeal to your mathematic side, you will be pleased by the focus on algebraic reasoning for shapes (section 8.3) and music (section 21.2), and a chapter on proof by induction (chapter 11).
After reading this book you will be prepared to take either of the two paths that Haskell is designed for: You can start writing useful and elegant tools, or you can dig into the fascinating programming language research going on. You will be prepared to approach arrows, a newer addition to Haskell which, like monads, have a deep relationship to category theory. Arrows are used extensively in some of the Yale Haskell group's recent work. You will see a lot of shared concepts between the animation in The Haskell School of Expression and Yale's "Functional Reactive Programming" framework, Yampa. If you like little languages, you'll appreciate how useful Haskell is for embedded domain-specific languages. It may be even more useful now that Template Haskell is in the works. Andrew Cooke described Purely Functional Data Structures as a great second book on functional programming. In my opinion, The Haskell School of Expression is the great first book you're looking for.
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I've read this book and think this is a good book for learning Haskell (perhaps the best one) and that it explains monads well.
However, it won't get the reader fully up and running as a productive Haskell programmer, because for that it is basically required that you master the GHC's (Glorious/Glasgow Haskell Compiler) standard library. Otherwise you won't know how to use a fast unboxed array, etc. This library is actually intelligently designed, but it is poorly documented, and there are lots of quirks for people who aren't totally familiar with Haskell yet. The best way to learn still seems to be to read the Haskell newsgroup and look at other people's code.
But Haskell is an extremely interesting language and well worth learning IMHO.
If you want to see some Haskell code, I have some more concrete examples here:
I have written a lot of little projects in Haskell. You can find some of them in links from my user info page.
Also, one of the best resources on Haskell is the HaWiki: HaWiki.
Do give Haskell a try. It is an amazing programming language.
Easy, automatic testing for Perl.
OCaml, which is not purely functional but still closely related to Haskell, is nearly as fast as C. Haskell somewhat acts as a testbed for ideas in the ML language family, and future versions of OCaml are expected to include many features that were first implemented in Haskell. I'd also suggest that Haskell is a good introductory language for future OCaml programmers as it ensures they won't just try and use OCaml like a weird imperative language.
OTOH, it is theoretically possible to automatically multithread purely functional programs, especially if they're lazy like Haskell. So it could end up being a very important language on multi-processor and distributed systems.
Finally, Haskell has an excellent foreign function interface for when you need C-like performance and control.
Above message is especially redundant, and not even funny. It's an exact duplication of a post from a previous functional programming thread on /.
You forget at least two things,
Functional languages are indeed used in production environments like Erlang from Ericsson for instance.
And there used to be Lisp machines.
So there are languages used in the "real" world and there "is/was" hardware available.
-- SouNerd.com
Short answer: IO is an exit value, just like you said.
Long answer:
Monads are a pattern for hiding a state that gets passed from a sequence of functions. For example, when you assign to a variable in an imperative language, the value of that variable in the implicit state is updated and all future phrases accessing that variable will get the new value. If you're using a Haskell state monad it works the same way, but you need to explicitly specify which phrases can be executed in the future (using sequencing operators much like C's semicolon). Effectively Haskell monads allow all imperative constructs except for backwards jumps (goto).
The Haskell IO Monad is a purely functional mechanism for ensuring that modifications of the RealWorld are done in the correct order. It is implemented to call system calls which have real side-effects, but wrapping IO in a monad ensures that you can still reason about the order the side-effects will occur in. Since Haskell is lazy, these side-effects won't actually be triggered until necessary, either because the program needs an input value or it exits.
(I can give examples if anyone wants, but the resources at haskell.org may be more helpful.)
Oddly enough...
I am working on a Haskell library that allows you to write haskell programs that output SWF files. It includes a scheme->SWF byte-code compiler so you can use scheme instead of action script.
Sorry, no URL.
Just as plenty of people are willing to put up with Python's slowness in exchange for better debugging, faster development, dynamic typing, etc., I think plenty of people would benefit by moving from C to Haskell, which is purely functional, has a great type inferencing system, never seg faults, etc.
One final note is that Haskell programs can often be optimized in Haskell itself and approach C speeds. This is because Haskell is compiled and statically typed and can deal with unboxed elements and so forth. This is a big difference from Python and other dynamically typed languages where optimization sometimes must be done in C for best results.
This is nonsense. Functional programming almost always requires less thought because:
They are garbage collected
Programs tend to be much shorted
There are fewer assignments
Type systems catch a huge number of stupid mistakes
Functional programming is only feels harder because your are forced to correct your errors (instead of silently failing).
It might be hard (only at first) to wrap your head around higher-order functions, but all of you java programmers do the same thing (with ugly syntax) when you create inner classes.
That is not true. If you were to transliterate this into caml, then the type of i would be a reference to foo, which g() could still change. Odds are though, you will very infrequently need to pass a reference.
Take a look at this one-page TCP port scanner that I wrote in Haskell. Imperative and functional styles mixed together, with neither sacrificing for the other.
To use your time- and frequency-domain metaphor, Haskell is the well-educated EE who can use both kinds of analysis -- and slide between the two with ease.
Easy, automatic testing for Perl.
Lots of other comments, as well as the story, have pointed to a number of good tutorials and introductions. I'd like to recommend also the one I wrote for IBM developerWorks. I believe my tutorial is a bit better for real beginners to FP than are most of the others out there.
Anyway, you can find it at IBM dW(free registration required) or at my own Gnosis Software site.
Buy Text Processing in Python
I guess this is a troll, but I can't resist:
Functional language are only good in theory.
This is totally not true. I build real programs that do real stuff in SML every day.
Thus, there's a MASSIVE performance loss when a functional programming language is executed on any of the existing processors.
This is also completely false. Optimizing high-level languages is often easier, because there is more semantic information to exploit (types, higher-order code). My SML programs typically run about 20% slower than a C counterpart, while being much shorter, more frequently correct, and more secure.
You might want to have a look at HaskellDB which is a Haskell library for writing statically checked queries using a relational algebra-like syntax. It lets you write things like:
Nice, isn't it?
I think the following should work:
:: (Array Integer Integer) -> (Array Integer Integer)
invertPermutation
invertPermutation ar =
let
swap (x,y) = (y,x)
in
array (bounds ar) (map swap $ assocs ar)
I'm not sure about the complexity of the function 'array' but I think that a good implementation could be linear.
It's not the imperative nature of the permutation algorithm that's magic; it's that C provides a primitive for (it seems) constant-time array access. Provide such a primitive in Haskell (see the Array class) and you have essentially the same linear-time algorithm. In fact, the Array section of the Haskell 98 report provides the following example:
I thought about coming up with a syllabus for myself of C, Haskell, LISP and Perl (which just evades me....)
I'd like to strongly recommend some books. The first is Modern C++ Design by Andrei Alexandrescu. The second is On Lisp by Paul Graham. In conjunction with that, you will need an introductory text on Lisp if you don't already know it and a good book on C++ templates. While I don't know what the best Lisp text currently in print is, I'd be willing to give Graham's ANSI Common Lisp a try on the strength of his other book. And C++ Templates: A Complete Guide by Vandevoorde and Josuttis illuminates a lot of dark corners in templates and explains some new features.
In the end, the goal is to understand how many times Graham and Alexandrescu are talking about the same things using very different languages. C++ templates are in many important ways a compile-time, functional macro language on top of C++ that implement many of the features of Lisp macros. For what it's worth, Bruce Eckel has mentioned this generic programming link in the context of discussing Java generics.
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