CODEWITHSUNDEEP
haskellhaskell-lens
Vagif Verdi
Vagif Verdi

Reputation: 4896

Getting Maybe from lens with default value

A lot of lens getters return Maybe values. And i often need to replace them with some default.

Say map lookup but with default.

fromMaybe "" $ Map.fromList [(1,"Foo")] ^? at 1

Can this be written with lens syntax? Maybe something close to this:

Map.fromList [(1,"Foo")] ^? at 1.or ""

Upvotes: 10

Views: 1783

Answers (1)

daniel gratzer
daniel gratzer

Reputation: 53881

I think what you want is

non :: Eq a => a -> Iso a (Maybe a)

non foo essentially does

 case someMaybe of
   Nothing -> foo
   Just a  -> a

In your case

 someMap ^. at 1 . non ""

Coincidentally, this is precisely the example of non the docs give.

If you want to use this with ix you're out of luck, but you can always do

 -- Import Data.Monoid
 defaulting :: a -> s -> Getting (First a) s a -> a
 defaulting a s fold = fromMaybe a $ s ^? fold

 foo = defaulting 0 [1, 2, 3] $ ix 3 -- 0

Upvotes: 10

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