Making it even less readable for some reason.

day7/main.exs

@@ -1,35 +1,31 @@
 defmodule Day7 do
   def run do
     lines =
-      File.read!("test.txt")
+      File.read!("data.txt")
       |> String.trim()
       |> String.split("\n")
       |> Enum.map(fn l -> String.split(l, ~r{\D+}) |> Enum.map(&String.to_integer(&1)) end)
       |> Enum.map(fn [head | tail] -> {head, tail} end)
 
-    [part1, part2] =
-      Enum.map([1, 2], fn part ->
-        # filter valid lines for parts 1 and 2, grab their targets, and sum them
-        Enum.filter(lines, &valid?(&1, part)) |> Enum.map(&elem(&1, 0)) |> Enum.sum()
-      end)
-
-    dbg({part1, part2})
+    [
+      [&*/2, &+/2], # part 1's operatons: * and +
+      [&*/2, &+/2, &("#{&1}#{&2}" |> String.to_integer())] # part 2's operatons: *, +, and concat
+    ]
+    |> Enum.map(fn ops ->
+      # filter valid lines for parts 1 and 2, grab their targets, and sum them
+      Enum.filter(lines, &valid?(&1, ops)) |> Enum.map(&elem(&1, 0)) |> Enum.sum()
+    end)
+    |> Enum.map(&dbg(&1))
   end
 
-  defp valid?({total, [a, b | rest]}, part) do
-    # if we're in part 1, just check a * b and a + b. in part 2, we'll also check a <> b
-    branches = if part == 1, do: [a * b, a + b], else: [a * b, a + b, concat(a, b)]
-
-    case [a, b | rest] do
-      # base case is 2 elements. if any succeed, return true
-      [_, _] -> Enum.any?(branches, fn n -> n == total end)
-      # recurse by trying each branch, subbing the first 2 elements with the new combined element
-      [_, _ | _] -> Enum.any?(branches, &valid?({total, [&1 | rest]}, part))
-    end
+  # base case, when there are 2 operands. returns true if applying any of the ops returns the target
+  defp valid?({total, [a, b]}, ops) do
+    Enum.any?(ops, fn op -> op.(a, b) == total end)
   end
 
-  defp concat(a, b) do
-    "#{a}#{b}" |> String.to_integer()
+  # apply each operation to the first two operands, prepend and recurse
+  defp valid?({total, [a, b | rest]}, ops) do
+    Enum.any?(ops, &valid?({total, [&1.(a, b) | rest]}, ops))
   end
 end