Background For the longest time I have been estranged to the magic lands of comprehensions in FP languages. Now I am trying to pick it up. Comprehensions use the for expression in Elixir: https://elixir-lang.org/getting-started/comprehensions.html#generators-and-filters How Scala does it Most F…

[image] Fl4m3Ph03n1x: By block, do you mean the do block, or the last generator of the expression? I mean the do block. The types of the generator sources don’t come into play. They all must be Enumerable and the comprehension … enumerates them. If it helps, you can envision a comprehension…

Sure, the :reduce option: for i <- 1..4, reduce: 0 do acc -> acc + i end (returns 10)

I am fairly aware I can force the return of the expression to be MapSet by using :into. However my objective is not to force a specific return type, is to understand how the returns type is picked.

But I am not completely sure what kind of expected output you’re after. Doing a combination of two or more collections can easily yield elements that don’t adhere to the rule of a MapSet (namely no repetitions). But maybe that’s exactly what you want? A collection with the products of two other coll…

Ah, I see you need an explanation of sorts. Oh well, I am just telling you what I’m seeing in the docs. You either return a list or a data structure you desire (:into and :reduce).

[image] dimitarvp: Can you show us your ideal return value for the scenario in your first post? One could, for example, based on the concept of familiarity, also expect that the return type of the Elixir comprehension is also the type of the first enumerator. So for example: for a <- [1, 2]…

As a sidenote I tried to search for for source code and it is a part of Kernel.SpecialForms and is implemented in bootstap module, kind of strange, maybe they did some optimizations.

[image] Fl4m3Ph03n1x: I am fairly aware I can force the return of the expression to be MapSet by using :into. However my objective is not to force a specific return type, is to understand how the returns type is picked. It’s always a list unless … you give it an :into which can be any Colle…

[image] gregvaughn: you give it a :reduce in which case it is whatever type your block returns on last iteration By block, do you mean the do block, or the last generator of the expression?

for, Stream and Enum are all based on the Enumerable protocol, which disregards the actual input datatype in favor of an unbounded enumeration of values. The enumerable implementations essentially turn the raw data into something users of the protocol can reduce over.

Can an elixir comprehensions return something other than a List?

Questions & Help Questions

dimitarvp February 15, 2022, 3:39pm 8

Yep, sorry, didn’t mean to pollute the thread.