Hi!
I have a Map and I want to replace the value associated to a key with
something that depends on that value, so that can only work if that value
does exist in the Map. For example:
~~~
map = %{1 => 2}
key = 1
Map.replace(map, key, map[key] + 2) # %{1 => 4}
~~~
Now, `key` is not known to me but I'm sure that if it's present in the
`map` I will be able to perform the operation on the value (so `map[key]`
will always be a number, in this example). If `key` is not present then I
will get an error:
~~~
map = %{1 => 2}
key = 3
Map.replace(map, key, map[key] + 2) # (ArithmeticError)
~~~
So I'm thinking maybe there should be Map.replace_lazy (there's already
Map.get_lazy, Map.pop_lazy and Map.put_new_lazy). I also checked other Map
functions that have a `fun` as a last argument and none allow to me to the
above. There's `Map.update(map, key, initial, fun)` but I don't want an
initial value, I just want to update the value if the key exists.
Another alternative it to let Map.replace be a macro so that the value can
be unquoted but only if the key is present.
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