Simplicity of pattern matching semantics, not of implementation (we
all knew how to implement it).
Miranda had non-linear patterns, but nobody really argued for them in Haskell.
If Haskell had them, I'd not argue to have them removed, but nor will
I argue to add them.
-- Lennart
On Fri, May 15, 2009 at 1:19 PM, Conor McBride
On 15 May 2009, at 12:07, Lennart Augustsson wrote:
In the original language design the Haskell committee considered allowing multiple occurrences of the same variable in a pattern (with the suggested equality tests), but it was rejected in favour of simplicity.
Simplicity for whom, is the question? My point is only that there's no technical horror to the proposal. It's just that, given guards, the benefit (in simplicity of program comprehension) of nonlinear patterns over explicit == is noticeable but hardly spectacular.
Rumblings about funny termination behaviour, equality for functions, and the complexity of unification (which isn't the proposal anyway) are wide of the mark. This is just an ordinary cost-versus-benefit issue. My guess is that if this feature were already in, few would be campaigning to remove it. (By all means step up and say why!) As it's not in, it has to compete with other priorities: I'm mildly positive about nonlinear patterns, but there are more important concerns.
Frankly, the worst consequence I've had from Haskell's pattern linearity was just my father's derision. He quite naturally complained that his programs had lost some of their simplicity.
All the best
Conor
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