smpoulsen / focus Goto Github PK

la = Lens.make_lens("a")
lb = Lens.make_lens("b")
i0 = Lens.idx(0)

This works
la ~> i0 ~> lb |> Lens.safe_view(%{"a" => [%{"b" => 3}]})

This crashes
la ~> i0 ~> lb |> Lens.safe_view(%{"a" => []})

** (FunctionClauseError) no function clause matching in Keyword.get/3
(elixir) lib/keyword.ex:150: Keyword.get([], 0, nil)
lib/focus.ex:147: anonymous fn/3 in Focus.compose/2

Shouldn't "safe_view" be safe and not crash when something is missing?

Hi !
just noticed that the second link in the References section (Lenses in Functional Programming) is broken

Is there any convention or best practice for using lens vs pattern-matching?

Hey, thanks for focus, this is quite superb way to deal with large payloads!

I have one question and I could not answer it after looking through Readme / codebase / tests.

Given the following datastructure:

payload = %{
items: [
  %{id: 1, price: 12.99}, 
  %{id: 2, price: 34.60}, 
  %{id: 3, price: 99.28}, 
]
}

how can one

• convert all ids to strings?
• collect all prices?
• round up prices?
• etc... ?

Does focus support targeting all items in a list in a generic way, means without explicit index option?

Thanks and have a nice day!
Roman

The original intention with making the lens the first argument was to support composing a long chain of lenses and piping into Focus.view/2, Focus.set/3, Focus.over/3 etc….

There is obvious value in making the data structure the first argument, as focus could then slot directly into pipeline operations.

iex(1)> Lens.make_lens(:id) |> Focus.set(%{id: nil}, 666)
{:error, {:lens, :bad_path}}
iex(2)> Lens.make_lens(:id) |> Focus.set(%{id: "foo"}, 666)
%{id: 666}

I've narrowed it down to this line. Is this intentional?

import Focus
data = %{a: []}
a = Lens.make_lens(:a)
b = Lens.make_lens(:b)
c = Lens.make_lens(:c)
d = Lens.make_lens(:d)
i0 = Lens.idx(0)

This works fine:
a ~> b ~> c ~> d |> Lens.safe_view(data)
{:error, {:lens, :bad_path}}

This crashes
a ~> i0 ~> b ~> c |> Lens.safe_view(data)
** (ArithmeticError) bad argument in arithmetic expression
Lensable.Tuple.getter/2
lib/lens/lens.ex:227: Lens.safe_view/2