[Haskell-cafe] Language extensions [was: Memoization]

[Philippa Cowderoy]

On Sun, 27 May 2007, Andrew Coppin wrote:

> Seriously. Haskell seems to attract weird and wonderful type system extensions
> like a 4 Tesla magnet attracts iron nails... And most of these extensions seem
> to serve no useful purpose, as far as I can determine. And yet, all nontrivial
> Haskell programs *require* the use of at least 3 language extensions. It's as
> if everyone thinks that Haskell 98 sucks so much that it can't be used for any
> useful programs. This makes me very sad. I think Haskell 98 is a wonderful
> language, and it's the language I use for almost all my stuff. I don't
> understand why people keep trying to take this small, simple, clean, elegant
> language and bolt huge, highly complex and mostly incomprehensible type system
> extensions onto it...
> 

Yeah, who needed type classes anyway?

By which I mean that that's always been the way with haskell, and once you 
get what the extensions do they tend to in fact be highly natural - 
sometimes to the extent that people forget that they were ever an 
extension (constructor classes, anyone?). 

For example, GADTs let you implement monads as interpreters by defining a 
datatype representing the abstract syntax tree that describes a 
computation - you can't get this to type without at a minimum existential 
types and for many monad operations you need the full power of GADTs to 
declare a corresponding constructor.

I imagine it would never have occurred to you to try implementing a monad 
that way, right? Similarly, a lot of the developments with type classes 
and polymorphism have been about letting people write sufficiently general 
libraries - they're driven by the demands of code that people want to 
write, but often not so much by the demands of single, simple 
applications.

Incidentally, Haskell 98 isn't that small a language itself - there's 
plenty of sugar around.

-- 
flippa at flippac.org

There is no magic bullet. There are, however, plenty of bullets that
magically home in on feet when not used in exactly the right circumstances.