CODEWITHSUNDEEP
haskellhaskell-lensoption-typelenses
ais
ais

Reputation: 2624

How to use lens set function with Maybe?

I have a tuple x and a Maybe value y

x = (1,1)
y = Just 2

I can do this

z = maybe x (\v -> x & _1 .~ v) y

Or I can create my own operator

(.~?) x y =  x %~ (\v -> fromMaybe v y)

z = x & _1 .~? y

But if lens doesn't have such operator, maybe I don't need it?

So, how to use lens set function with Maybe?

Upvotes: 2

Views: 741

Answers (1)

dfeuer
dfeuer

Reputation: 48591

It appears that you want

maybeSetFst :: (a, b) -> Maybe a -> (a, b)

which will update the first field if given an update value and will leave it alone otherwise. I think the first implementation you give is very good, but you can give it a more general type:

maybeSetFst :: Field1 s s a a => s -> Maybe a -> s

If you don't want that generality, you can skip the lenses and write (using TupleSections)

maybeSetFst p@(_,b) = maybe p (,b)

Another option is to apply maybe to get the update function:

maybeSetFst p m = maybe id (_1 .~) m p

which can be written

maybeSetFst = flip $ maybe id (_1 .~)

for point-free silliness.

Upvotes: 2

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