[Haskell-cafe] Re: GHC predictability

Anton van Straaten anton at appsolutions.com
Tue May 13 15:38:51 EDT 2008

Achim Schneider wrote:
> To get a bit more on-topic: I currently completely fail to implement a
> layout rule in Parsec because I don't understand its inner workings,
> and thus constantly mess up my state. Parsec's ease of usage is
> deceiving as soon as you use more than combinators: Suddenly the
> plumbing becomes important, and hackage is full of such things. Haskell
> has potentially infinite learning curves, and each one of them
> usually represents a wall. To make them crumble, you have to get used to
> not understand anything until you understand everything.

A big component of this is just that a high level of abstraction is 
involved.  Something similar occurs in other languages, for programs 
that are written in a very abstract way.  Some frameworks in e.g. 
Smalltalk, Java, or C++ are an example of this: full of classes whose 
domain is mainly internal to the framework, and you have to understand 
the framework's design principles in their full generality in order to 
be able to really understand the code.

As a more concrete example related to Parsec, consider a generator of 
table-driven parsers written in C, and compare this to writing a 
recursive-descent parser directly.  The code for the parser generator is 
completely impenetrable for someone who isn't familiar with the theory 
behind it, so if they want to change the generator's behavior, they're 
likely to be stuck.  Whereas for a recursive descent parser for a single 
language, it's much easier to map between the ultimate application 
goals, and how those are accomplished in the code, without much special 

Of course there are pros and cons on either side.  One reason that DSLs 
work well is that when done right, so that abstraction leakage is 
minimal, they can insulate users from having to understand the 
underlying system.  Embedded DSLs, like Parsec, seem more likely to 
suffer from problems in this area, although in that case the tradeoff is 
that you're getting to use them directly in a general-purpose language.


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