Merging Dict and NamedTuple · helpdesk (published)

let
    a = (a=1, b=2)
    b = Dict(:c=>3, :d=>4)
    merge(a, b)
end

(a = 1, b = 2, d = 4, c = 3)

but merge(b, a) fails with MethodError. Is the method simply not implemented, or am I missing why it's a bad idea?

jar (Mar 21 2022 at 22:56):

mbaz (Mar 21 2022 at 22:59):

peter j. (Mar 22 2022 at 00:45):

mbaz (Mar 22 2022 at 01:55):

let
    a = (a=1, b=2)
    b = Dict(:c=>3, :d=>4)
    merge(NamedTuple(b), a)
end
(d = 4, c = 3, a = 1, b = 2)

Sundar R (Mar 31 2022 at 19:33):

I don't clearly see how this leads to merge(::Dict, ::NamedTuple) not being defined while the other way around is defined.

merge docs don't mention any asymmetry regarding key types - there's only "If necessary, the types of the resulting collection will be promoted to accommodate the types of the merged collections".

And if there is asymmetry, my mental model would be that the first argument's key type would be the "primary" one that the rest of them should adhere to - so if anything, Dict being the second argument should be the problematic case (if we're assuming asymmetry at all).

I think (unless I'm missing something) it's just a missing implementation - something like merge(itr, a::NamedTuple) = merge(a::NamedTuple, itr).

jar (Mar 31 2022 at 20:15):

I think this operation doesn't make sense. getindex and getproperty are different projectors. There could be a type that has both, with different values for the same key.

jar (Mar 31 2022 at 20:19):

I'm not so sure about the idea behind merge or even pairs/keys/values really.

mbaz (Mar 31 2022 at 21:35):

@Sundar R How could Dict("1"=>"V") be converted to a NamedTuple? A NamedTuple restricts the type of the keys, whereas a Dict doesn't.

mbaz (Mar 31 2022 at 21:41):

@Sundar R Maybe what merge should do is not to promote the keys, but to promote the resulting container so that it can accomodate all k,v pairs being merged.

mbaz (Mar 31 2022 at 21:43):

merge(a::NamedTuple, iterable)

  Interpret an iterable of key-value pairs as a named tuple, and perform a merge.

which seems to imply any iterable can be merged into a NamedTuple, but that's not the case:

merge((a = true,), Dict("1" => 1.5))
ERROR: TypeError: in typeassert, expected Symbol, got a value of type String

mbaz (Mar 31 2022 at 21:45):

This fails because it wants to return a NamedTuple, but it should (?) return a Dict.

mbaz (Mar 31 2022 at 21:48):

@jar Why do you think merge makes no sense? Sometimes you have two containers and want to combine them.

Zachary P Christensen (Mar 31 2022 at 21:55):

Hit this recently and thought it was weird that it didn't just return a dict too.

Sundar R (Mar 31 2022 at 22:00):

Yeah, I understand that part, but if merge is symmetric between its arguments (i.e. treats them the same), then it shouldn't matter which order we send a Dict and NamedTuple in: either both merge(::Dict, ::NamedTuple) and merge(::NamedTuple, ::Dict) work, or neither does. Passing a non-Symbol keyed Dict as second argument currently throws a TypeError, and with merge(itr, a::NamedTuple) = merge(a::NamedTuple, itr) it would do the exact same for the other way around too.

It's weird to treat these cases differently and give different errors, which is what leads me to think it's an implementation oversight and not a conscious design decision.

mbaz (Mar 31 2022 at 22:05):

Yeah -- the order of the arguments might be relevant (as it is now) to decide what to do in case of repeated keys, but not to decide on the type of the result.
I wonder if it's worth it to open an issue about it.

Stream: helpdesk (published)

Topic: Merging Dict and NamedTuple

mbaz (Mar 21 2022 at 22:33):