Lenses and Nested Maps

[Henry Laxen]

Hi Cafe,

Suppose I have:

s :: Map String (Map Int Char)
s =  M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]
     
Is there any way to modify the 'b' entry easily using lenses?

Thanks in advance.
Best wishes,
Henry Laxen

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[Michael Sloan]

Hi Henry,

Yes, the "ix" function traverses into particular keys of map-like
structures https://www.stackage.org/haddock/lts-13.25/lens-4.17.1/Control-Lens-At.html#v:ix

So, (s & ix "bob" . ix 6 .~ 'c') will yield a modified Map with 'b'
changed to 'c'.

-Michael

On Mon, Jun 17, 2019 at 9:45 PM Henry Laxen <[hidden email]> wrote:

>
> Hi Cafe,
>
> Suppose I have:
>
> s :: Map String (Map Int Char)
> s =  M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]
>
> Is there any way to modify the 'b' entry easily using lenses?
>
> Thanks in advance.
> Best wishes,
> Henry Laxen
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.

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[David Feuer]

The main lensy tools for maps are the At and Ixed classes, both found in Control.Lens.At.

On Mon, Jun 17, 2019, 11:45 PM Henry Laxen <[hidden email]> wrote:

Hi Cafe,

Suppose I have:

s :: Map String (Map Int Char)
s =  M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]

Is there any way to modify the 'b' entry easily using lenses?

Thanks in advance.
Best wishes,
Henry Laxen

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[Jack Kelly]

Henry Laxen <[hidden email]> writes:

> s :: Map String (Map Int Char)
> s =  M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]

`ix` will give you a `Traversal` that targets a given key. If it's not
present in the map, you get a `Traversal` that hits no targets.

t :: Map String (Map Int Char)
t = s & ix "bob" . ix 6 %~ succ

*Main> t
fromList [("alice",fromList [(5,'a')]),("bob",fromList [(6,'c')])]

If you want more control (to set or delete values, for instance), the
`at` function gives you a lens that views a `Maybe`. Writing `Just` into
the lens creates the value at the key, and writing `Nothing` into the
lens removes the value at the key. The `(?~)` operator, and the `_Just`
prism may be useful to you if you use `at`.

HTH,

-- Jack
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