Douglas,<br><br>Excellent questions you posed to Simon P-J -- who then forwarded them to the Haskell Cafe list. By way of answering i should say i was a Schemer from the get-go; it was really the first programming language i studied as an undergraduate majoring in maths at Oberlin in the early 80's. Eventually, i went on to design and build my own language (at MCC with Christine Tomlinson, the principal investigator) called Rosette. While Scheme was Sussman and Abelson's way of making sense of Hewitt's ideas in a sequential setting Rosette was our way of doing the full banana -- including the actor-based form of concurrency as well as both structural and 3-Lisp-style procedural reflection and a whole host of other advanced features. So, i was naturally profoundly frustrated when the world at large turned to languages like C, C++ and even Java. i have been waiting more than 20 years for the industry to catch up to the joys of advanced language design. <br clear="all">
<br>Now that the industry has taken a shine to functional languages again i have been spending more time with the various modern flavors and have to say that while each of the major contenders (ML, OCaml, Erlang, Scala, Haskell) have something to be said for them, Haskell stands out among them. Haskell enjoys a particular form of mental hygiene that few other languages enjoy. Its syntax, by comparison with Scheme, is remarkably concise -- and, the importance of syntax is almost impossible to gauge because at the end of the day it is code one is staring at ;-). The chief semantic differences that make a difference to my mind may be classified as follows.<br>
<ul><li>types</li><li>monads</li><li>meta-programming</li></ul>In order, then: at its outset Haskell made a strong commitment to a potent (static) typing scheme. Even if types can be layered on Scheme, the two language design vectors are remarkably different and give programming a different feel. As a result of my academic and professional training i have come to rely heavily on types as a development discipline. In fact, if i cannot devise a sensible type "algebra" for a given (application) domain then i feel i don't really have a good understanding of the domain. One way of seeing this from the Schemer point if view is that the deep sensibility embodied in the Sussman and Abelson book of designing a DSL to solve a problem is further refined by types. Types express an essential part of the grammar of that language. In Haskell the close connection between typing and features like pattern-matching are also part of getting a certain kind of coherence between data and control. Again, this can be seen as taking the coherence between data and control -- already much more evident in Scheme than say C or C++ or even Java -- up a notch.<br>
<br>Haskell's language-level and library support for monads are really what set this language apart. i feel pretty confident when i voice my opinion that the most important contribution to computing (and science) that functional programming has made in the last 15 years has been to work out practical presentations of the efficacy of the notion of monad. As a Schemer i'm sure you understand the critical importance of composition and compositional design. The monad provides an important next step in our understanding of composition effectively by making the notion parametric along certain dimensions. This allows a programmer to capture very general container patterns and control patterns (as well as other phenomena) with a very concise abstraction. Again, this could be layered onto Scheme, but Haskell embraced monad as a central abstraction at the language design level and this leads to very different approaches to programming.<br>
<br>Now, the place where Haskell and the other statically typed functional languages have some catching up to do is meta-programming. Scheme, Lisp and other languages deriving from the McCarthy branch of the investigation of lambda-calculus-based programming languages enjoy a much longer and deeper investigation of meta-programming constructs. While MetaOCaml stands out as a notable exception i think it safe to say that 3-Lisp and Brown are pretty strong evidence of the long history and much richer investigation of meta-programming notions along the McCarthy branch than along the Milner branch. The industry as a whole, i think, has embraced the value of meta-programming -- witness (structural) reflection in such mainstream languages as Java and C#. And the Milner branch family of languages are moving rapidly to catch up -- see the efforts on generic programming like S P-J's SYB or TemplateHaskell -- but the deep coherence evident in the simplicity of the monadic abstraction has not met up with the deep coherence of 3-Lisp, yet.<br>
<br>Anyway, that's my two cents... but i note that US currency is not worth what it used to be.<br><br>Best wishes,<br><br>--greg<br><br>
Message: 21<br>
Date: Tue, 1 Apr 2008 11:18:25 +0100<br>
From: Simon Peyton-Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>><br>
Subject: [Haskell-cafe] FW: Haskell<br>
To: Haskell Cafe <<a href="mailto:haskell-cafe@haskell.org">haskell-cafe@haskell.org</a>><br>
Cc: "<a href="mailto:kerrangster@gmail.com">kerrangster@gmail.com</a>" <<a href="mailto:kerrangster@gmail.com">kerrangster@gmail.com</a>><br>
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<br>
Dear Haskell Cafe members<br>
<br>
Here's an open-ended question about Haskell vs Scheme. Don't forget to
cc Douglas in your replies; he may not be on this list (yet)!<br>
<br>
Simon<br>
<br>
-----Original Message-----<br>
From: D. Gregor [mailto:<a href="mailto:kerrangster@gmail.com">kerrangster@gmail.com</a>]<br>
Sent: 30 March 2008 07:58<br>
To: Simon Peyton-Jones<br>
Subject: Haskell<br>
<br>
Hello,<br>
<br>
In your most humble opinion, what's the difference between Haskell and<br>
Scheme? What does Haskell achieve that Scheme does not? Is the choice less<br>
to do with the language, and more to do with the compiler? Haskell is a<br>
pure functional programming language; whereas Scheme is a functional<br>
language, does the word "pure" set Haskell that much apart from Scheme? I<br>
enjoy Haskell. I enjoy reading your papers on parallelism using Haskell.<br>
How can one answer the question--why choose Haskell over Scheme?<br>
<br>
Regards,<br>
<br>
Douglas<br>
<br><br>-- <br>L.G. Meredith<br>Managing Partner<br>Biosimilarity LLC<br>806 55th St NE<br>
Seattle, WA 98105<br><br>+1 206.650.3740<br><br><a href="http://biosimilarity.blogspot.com" target="_blank">http://biosimilarity.blogspot.com</a>