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add function fmapNonNull #150

cdepillabout opened this issue Dec 13, 2017 · 2 comments


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commented Dec 13, 2017

When I am working with NonNull in a new codebase, I often reimplement the following function:

  :: (Functor f, MonoFoldable (f b))
  => (a -> b) -> NonNull (f a) -> NonNull (f b)
fmapNonNull f = impureNonNull . fmap f . toNullable

This lets you call fmap on the underlying container in a NonNull. Here's an example:

> let nonNullable = impureNonNull [1,2,3] :: NonNull [Int]
> fmapNonNull show nonNullable :: NonNull [String]

This is not possible with omap, because it does not let you change the type of the element in the underlying container.

I think this fmapNonNull would be widely useful, because NonNull is often used with Functors like [] or Seq.

This function could go into the Data.NonNull module.

Here are some possible names for this function:

  • fmapNonNull
  • fmapNonNullable
  • mapNonNull
  • mapNonNullable

I think any of those four names would be suitable.

If the maintainers agree that this could go into Data.NonNull, I will send a PR to include it.


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commented Dec 14, 2017


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commented Dec 19, 2017

Added in #151.

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