~ patterns

[Patryk Zadarnowski]

On 31/01/2006, at 9:31 PM, Simon Marlow wrote:

> We must find *something* to throw away though! :-)

One small issue that I'd love to see thrown away is the special  
handling the the unary "-"
operator in Haskell 98. It's been described as "embarrassing",  
"ugly", and even "inconvenient"
I, for one, find myself creating sections of the form `+ (-x)` all  
the time, and it feels wrong.

The proposal would be to remove the unary "-" altogether, and,  
instead, extend the lexical
syntax of numeric constant to allow "+" and "-" prefix. Further, we  
would need to extend
the prelude with an additional definition:

	negate :: Num a -> Num a
	negate x = 0 - x

Pros:

1. Removes an embarrassing special case from the grammar.
2. Makes the section `- x` work as expected.
3. Expressions such as "-1" would not require paranthesizing.
4. Expressions such as "-1" would be permitted in k-patterns
     even if n+k patterns end up being thrown out.
5. You can say "negate $ 1 + 2" if you don't like parentheses.
6. The precedent of making an operator symbol behave differently when
     not separated from its argument by a space has already been made by
     the "." operator.

Cons:

1. Expressions of the form "-x" (where "x" is not a constant) need to  
be rewritten
     as "negate x" (which, to me, looks much cleaner anyway,  
especially as, more
     often than not, "x" ends up being a complex expression anyway!

2. Possible confusion to the beginners (can write "-1" but cannot  
write "-x".)
     However, I think that the strange behaviour of sections and the  
need for
     parentheses around "-1" is already confusing enough to  
beginners, and
     therefore this chance would actually make Haskell *easier* to  
learn, not
     harder. "negate x" looks so much more like ordinary Haskell code!

What do people think?

	- Pat.