[Haskell-cafe] about Haskell code written to be "too smart"

wren ng thornton wren at freegeek.org
Tue Mar 24 23:32:30 EDT 2009

Manlio Perillo wrote:
> But this may be really a question of personal taste or experience.
> What is more "natural"?
> 1) pattern matching
> 2) recursion
> or
> 1) function composition
> 2) high level functions

Which is more "natural":
* C-style for-loops (aka assembly while-loops), or
* any modern language's foreach loops (aka iterators)?

Following directly from the Rule of Least Power, if you can get away 
with foreach then that's what you should use. Why? Because the less 
power the construct has, the fewer corner cases and generalizations a 
reader of the code needs to consider. Now, just because iterators exist 
does not mean that one should never use the more general tool. If you're 
fighting to break out of your chosen straitjacket, then chances are it's 
the wrong one to use in the first place; it'd be clearer to use more 
power and have less fighting.

Both of these conclusions seem quite natural to me, even from before 
learning Haskell. It seems, therefore, that "naturality" is not the 
proper metric to discuss. It's oft overlooked, but the fact is that 
expressivity comes not from more formal power, but from _less_.

* A human's (or any vertebrate's) range of motion is severely crippled 
when compared to that of an amoeba; and yet it is those limitations 
which provide the structure necessary to perform greater tasks such as 
grasping, lifting, jumping, etc.

* Natural language has a limited range of words and syntactic 
constructs, but gives the larger-enough building blocks to enable 
unconstrained communication; whereas a language with a unique word for 
every utterance (arguably simpler) is impossible to learn.

* Regular expressions (and other classes of automata) have severe 
limitations on formal power, and yet these constraints enable poly-time 
algorithms for intersection, union, etc.

* Haskell's type system (sans extensions) is not Turing complete, yet 
this enables us to infer types rather than requiring annotations or proofs.

The contemporary state of scientific research is focused heavily on the 
idea of reductionism (the idea of being able to reduce all biology to 
chemistry, all chemistry to physics, all computer science to 
mathematics, etc). But as any systems theorist will tell you, this 
approach is misguided if the goal is a Theory of Everything. As per the 
famous book: no matter how much you learn about quarks, that tells you 
nothing about jaguars.

At every step of reduction, there is an increase in formal power and a 
concomitant loss of information. Even perfect knowledge of quarks and 
perfect simulation software isn't enough, because you've lost the 
_abstraction_ that is "jaguar". You can simulate it, emulate it, model 
it, but you've lost the high-level perspective that says jaguars are 
different and more interesting than an arbitrary simulation of a 
collection of quarks. (And it's doubtful we'll ever have the omniscience 
to get even that far.)

While primitive recursion and case matching are _fundamental_ (that is, 
at the bottom of a reductionist tower), that does not entail that they 
are _central_ (that is, a ubiquitous pattern at every resolution of 
reduction). Church encoding, SKI combinators, Curry-Howard isomorphism, 
and the like are also fundamental topics to teach and understand; but 
they're rarely ones that should be central to a program or library.

Now, many Haskellers (like good scientists) bristle at this fundamental 
nature of things. And in response we're constantly coming up with new 
generalizations which have little-enough structure to be descriptive 
while having big-enough structure to be interesting. If there's too much 
structure, it's boilerplate and therefore unusable; if there's too 
little, it has no generality and is therefore unhelpful. But somewhere 
between those extremes someone has to make a judgment call and decide 
whether some particular pattern measures up to the metric of being 
helpful and usable. If it does, then everyone (whose domain it covers) 
should learn it and use it because it simplifies programming from a 
high-level of design.

Giants. Shoulders. Etc.

Live well,

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