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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