[Haskell-cafe] Re: How Albus Dumbledore would sell Haskell

Derek Elkins derek.a.elkins at gmail.com
Thu Apr 19 23:54:31 EDT 2007

DavidA wrote:
> Simon Peyton-Jones <simonpj <at> microsoft.com> writes:
>> But, just to remind you all: I'm particularly interested in
>>   concrete examples (pref running code) of programs that are
>>        * small
>>        * useful
>>        * demonstrate Haskell's power
>>        * preferably something that might be a bit
>>                tricky in another language
> I have something that I think nearly fits the bill. Unfortunately, I don't 
> think it quite works because it's a bit specialised. However, I think it 
> suggests a possible area to look, which I'll mention at the end.
> It's a theorem prover for intuitionistic propositional logic:
> http://www.polyomino.f2s.com/david/haskell/gentzen.html
> It's much shorter in Haskell than it would be in other languages. (It's even 
> shorter than the ML that I based it on, because of some shortcuts I can take 
> using lazy evaluation.)
> Strengths of Haskell that it demonstrates are:
> * How easy it is to define datatypes (eg trees), and manipulate them using 
> pattern matching, with constructors, Eq, Show coming for free.
> * How lazy evaluation reduces code length by letting you write code that looks 
> like it would do too much, and then lazy evaluate it (in the "proof" function)
> * The ability to extend the syntax with new symbolic operators
> * Use of higher order functions to simplify code (the (+++) operator)
> The problem is that I think Gentzen systems are a bit obscure. But I think you 
> could probably show most of the same strengths of Haskell in something 
> similar: game search, eg alpha-beta algorithm. Another advantage of doing game 
> search would be that you'd get to show off persistent data structures (so that 
> when you make a move in lookahead, you don't need to make a copy of the game 
> state, because when you update the game state the old state still persists).

Game search is exactly an example use in "Why Functional Programming Matters" 
(http://www.math.chalmers.se/~rjmh/Papers/whyfp.html).  That paper, 23 years 
later, is still pretty compelling.  Perhaps, it should just be modernized and 
somewhat expanded.

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