Comprehensions in Elixir

for generates

The for special form builds collections from generators, running its body once per drawn value. Read the generator arrow as drawn from, as in each n drawn from the list.

for n <- [1, 2, 3] do n * 10 end

Each list element flows into the body and the results gather into a fresh list. One generator keeps things simple, while several generators multiply into every combination.

Filter with guards

Guards after the generator keep only the values you want, acting as a filter built into the loop. The body runs solely for values that satisfy every guard.

for n <- [1, 2, 3, 4], rem(n, 2) == 0 do n * 100 end

Odd numbers fail the remainder test and vanish, so only 2 and 4 reach the body and return 200 and 400. Chain several comma separated guards to express tighter selection rules.

Collect with into

By default for collects into a list, but the into option redirects results into any collector. Maps, keyword lists, and even binaries accept the generated values.

for n <- [1, 2, 3], into: %{} do {n, n * n} end

Each pair becomes one map entry, giving squares keyed by their roots. Reach for into whenever the caller wants a map instead of a list of tuples.

Bitstring generators note

Generators also draw from binaries, matching each byte segment in turn. This turns for into a compact tool for text scans and small protocol parsers.

for <<c <- "hi">>, into: "" do <<c + 1>> end

Each character code grows by one and the into option rebuilds a binary, shifting hi to ij. Binary generators deserve care with multi byte text, where String tools handle graphemes more kindly.

Next: Advanced strings in Elixir

Article author: Arthur Isaev