Dict vs Record in elm

Question

While implementing a simple app I ran into the problem of trying to update a nested record. I found a solution online but it really seems like a whole lot of bloated code.

As I was looking for alternatives I found Dictionaries. This seem like a solution to that problem -- If I use a dictionary inside of a record I can avoid all that bloated code and get nested updates.

Seeing dictionaries and records next to each other made me wonder, why would I use a record instead of a dictionary, or vice versa? The two seem really similar to me, so I am not sure I see the advantage of one or the other. Of course I can see that there is a difference in syntax, but is that all ?

I learned somewhere that the access time complexity of Dict is O(log(n)) -- does it do a binary search on the keys ? -- but I can't find the access time complexity for record, but I am wondering if that is O(1) and that is one of the advantages.

Either way, they both seem to map to 1 single data structure in other languages (e.g Python's dictionaries, JS objects, Java hash-tables), why do we need two in elm ?

Answer 1

Dicts and records might seem very similar when coming from JavaScript, but in a statically typed language they are actually very different. I think just about the only property they have in common is that they are both key-value containers.

The biggest differences, I think, are that Dicts are homogeneous, meaning values must be of the same type, and "dynamically" keyed and sized, meaning keys are not statically checked (ie. at compile-time) and that key-value pairs can be added at runtime. Records on the other hand includes the key names and value types in the record type, which means they can hold values of different types, but also can't have keys added or removed at runtime without changing the type itself.

The benefits of easily being able to insert and update a Dict is something you pay for when you try to get it back out. Dict.get returns a Maybe which you'll then have to handle, because the type doesn't give any guarantee that it contains anything at all. You also won't get a compiler error if you mistype the name of a key.

Overall, a Dict forsakes most of the benefits of static typing. I think a good rule of thumb is that if you know the key names, you should most likely go with records. If you don't, go with Dict.

You also seem about right regarding performance, but I think that's a secondary concern. Record access should be equivalent to accessing the elements of an array by index, since so much information is known at compile time that it can essentially be compiled down to a fixed-size array.