CODEWITHSUNDEEP
haskell
gnuvince
gnuvince

Reputation: 2387

incorrect lens definition in Edward Kmett's presentation?

I'm slowly trying to wrap my head around how the lens package works. I'm reading Edward's slides (http://comonad.com/haskell/Lenses-Folds-and-Traversals-NYC.pdf) and I have a question about the definition of Store s a on slides 8 and 9:

newtype Lens s a = Lens (s -> Store s a)
data Store s a = Store (s -> a) s

Is the definition backwards? If lenses are pairs of getters and setters, it seems to me that Store s a should be defined as:

data Store s a = Store (a -> s) a

where the first constructor parameter is the setter (put an a and return the s data structure) and the second is the getter (fetch the a from the s data structure).

Can anyone confirm if it's an error in the slides, or if my understanding is incorrect, please explain.

Thank you.

Upvotes: 1

Views: 227

Answers (1)

shachaf
shachaf

Reputation: 8930

Yes, that looks like an error. But Store is correct -- it's a standard comonad. The definition of Lens should be newtype Lens s a = Lens (s -> Store a s).

Upvotes: 5

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