Hi all, recently i have been trying out for comprehension after reading Introducing `for let` and `for reduce` and realizing i am also have severly underutilized for. Just now i have spend quite sometime debugging code written with for only to realize there is the following: there’s no “only” binding operation, it always also get used with filters.

For example, consider following code:

data = [%{a: 1, b: 2}, %{a: 2}]

for datum <- data,
  b = datum[:b],
  b == nil do
    datum
end

what would you expect the result be? [%{a: 2}] like i was? the actual result (on elixir 1.10.3) was []. b = datum[:b] is actually treated as filter, and elixir recognize nil and false as falsey value.

I think this is pretty strange behaviour, though maybe this is the implication on A comprehension is made of three parts: generators, filters, and collectables. from https://elixir-lang.org/getting-started/comprehensions.html ?

For context, even at the web tutorial we find following code, in which path = Path.join(dir, file) is treated as variable assignment.

dirs = ['/home/mikey', '/home/james']

for dir <- dirs,
    file <- File.ls!(dir),
    path = Path.join(dir, file),
    File.regular?(path) do
  File.stat!(path).size
end

and at Haskell, they have separation between declaration and filter, like in following

image1627×236 17.7 KB