On Fri, Sep 10, 2010 at 07:51:10PM +0200, S. Doaitse Swierstra wrote:
>
> Currently Haskell has infix, infixl and infixr operators. I see a use for
> infixlr as well. This indicates that the implemtation may assume the operator
> to be associative, and thus has the freedom to "balance" an expression
> containing several operator occurrences.
Would it be restricted to use with operators with types that are (a -> a
-> a) (or more specific)?
Otherwise e.g.
let (+:) = (:)
infixlr :+
in [] +: [] +: []
could have type [[a]] or [[[a]]].
> The reason that I bring up this is that in a new combinator I have added to
> my parser library (the <||> in Text.ParserCombinators.UU.Derived) internally
> uses cartesian products, which are being constructed and updated. If the
> compiler had the right to interpret the expressions a <||> b <||>c <||> d
> as e.g. (a <||> b) <||> (c <||> d) then the updating time for would go down
> from O(n) to O(log n).
How would the compiler work out which parsing to prefer? Or would it
assume that infixlr expressions are best balanced?
When first reading the proposal, I thought the idea was to allow the
compiler to more easily perform optimisations like
a+b+c+2+3+d => a+b+c+5+d
but I guess that wasn't something you were thinking about?
Thanks
Ian
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