[Haskell-cafe] Problems with "Bananas"

Creighton Hogg wchogg at gmail.com
Sat Apr 19 20:37:50 EDT 2008

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.

I'm sure I'm just being dumb, but this is really bugging me.

Creighton Hogg
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