[Haskell-cafe] All equations must have the same arity - why?

[Roman Leshchinskiy]

Neil Mitchell wrote:
> Hi,
> 
> It's nice to write functions in point free style:
> 
> f = sort . nub
> 
> But sometimes I have to add an extra case, on a certain value:
> 
> f [] = [1]
> f = sort . nub
> 
> But now these equations have different arities, and its rejected by
> Haskell. Why does this not simply desugar to:
> 
> f [] = [1]
> f x = (sort . nub) x
> 
> i.e. lift the arities to the longest argument list.
> 
> Is there a reason this isn't done?

In addition to the sharing problem, consider

f True x = x
f False  = undefined

Because of seq, the second equation is not equivalent to the eta-expanded

f False x = undefined x

Roman