[Haskell-cafe] Problems with "Bananas"

Ross Paterson ross at soi.city.ac.uk
Sat Apr 19 21:00:59 EDT 2008

On Sat, Apr 19, 2008 at 07:37:50PM -0500, Creighton Hogg wrote:
> This isn't about Haskell per se, but I was reading the old Meijer et al. paper 
> "Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire" & I
> think there's a notational pun that's really confusing me.
> On page 12 we have the CataEval equation
> (|phi|) . in = phi . (|phi|)_L
> Now, the subscript L & the following example of cons lists implies that L is a
> functor in this equation, yet the line immediately after this equation says
> that "(CataEval) states how to evaluate an application of (|phi|) to an
> arbitrary element of L...." so then that makes it sound like the L here is the
> fixed point of some functor F, not the functor itself.

Yes, the explanatory text should say mu L instead of L.

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