Re: Haskell 98 -- Overloading

[S. Alexander Jacobson]

I guess I missed the controversy over at ICFP, but I would like to know
why overloading of lists is being eliminated.

Arguments for Overloading:
1. Generality/Re-use is good
The big point of Hughes "Why Functional Progamming Matters" is that
functional programming enables much more high level notions of re-use and
that is a good thing.  If you eliminate overloading of the ++ operator
then it is much less straightforward to replace code which uses Lists 
as a collection type with code that uses Trees as its collection type.
Now that you are adding MPTC, the argument in favor or retaining ++ as an
overloaded operator is even stronger.  It would be a pity to have to go
through that code, find every instance of ++ and replace it with mplus.  
It makes more sense to encourage programmers to use reusable constructs
from day 1.  The most reusable constructs are the overloaded operators.

2. Provide a path for beginners to learn the generalization
The overloading constructs provide a path for a newby (like me) to learn
how to generalize concepts like e.g. Lists to arbitrary monads (still
working on this one).  Eliminating overloading makes it that much harder
for someone who has learned Haskell basics to move from understanding ++
as the concatentation operator to understanding that  xs ++ ys really
means "the content of xs" orelse "the content of ys".  This generality
allows me to go from understanding ++ as list concatenation to
understanding ++ as collection type combination to understanding ++ as
describing a certain type of computation (the backtracking monad).

--
>From the discussion of Integer vs. Int, it seems like the best objection
to overloading is that error messages are hard for beginners to
understand.  This doesn't strike me as an argument against overloading but
rather an argument in favor of a better error reporting mechanism.  
I am not sure how it would work, but I think that one could construct an
error reporting mechanism with special purpose errors for special purpose
uses that are common.  E.g. a type error involving use of lists and ++
might generate a different error message from the same type error
involving a less common type.

-Alex-

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S. Alexander Jacobson                   i2x Media  
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