Clarify semantics of defguard, closes #15284

Author: josevalim

lib/elixir/lib/kernel.ex

@@ -5873,43 +5873,46 @@ defmodule Kernel do
   end
 
   @doc """
-  Defines a macro suitable for use in guard expressions.
+  Defines a custom guard with the given name.
 
-  It raises at compile time if the `guard` uses expressions that aren't
-  allowed in [guard clauses](patterns-and-guards.html#guards),
-  and otherwise creates a macro that can be used both inside or outside guards.
+  Once defined, custom guards can be invoked within regular code or in
+  guards. The module that contains the custom guard must be required before usage.
+
+  Custom guards are defined by providing a valid guard expression to
+  the right-hand side of `when`. `defguard` will then expand and validate
+  the expressions as guards. `defguard` will raise at compile time if the
+  guard uses expressions that aren't allowed in [guard clauses](patterns-and-guards.html#guards).
 
   When defining your own guards, consider the
   [naming conventions](naming-conventions.html#is_-prefix-is_foo)
   around boolean-returning guards.
 
   ## Example
 
+  For example, to define a guard similar to `Integer.is_even/1`, you can write:
+
       defmodule Integer.Guards do
         defguard is_even(value) when is_integer(value) and rem(value, 2) == 0
       end
 
-      defmodule Collatz do
-        @moduledoc "Tools for working with the Collatz sequence."
-        import Integer.Guards
+  which can then be used as:
 
-        @doc "Determines the number of steps `n` takes to reach `1`."
-        # If this function never converges, please let me know what `n` you used.
-        def converge(n) when n > 0, do: step(n, 0)
+      require Integer.Guards
+      Integer.Guards.is_even(3)
+      #=> false
 
-        defp step(1, step_count) do
-          step_count
-        end
+  ## Implementation details
 
-        defp step(n, step_count) when is_even(n) do
-          step(div(n, 2), step_count + 1)
-        end
+  Behind the scenes, `defguard` will generate a macro which can be used
+  inside and outside of guards, preserving their respective semantics.
 
-        defp step(n, step_count) do
-          step(3 * n + 1, step_count + 1)
-        end
-      end
+  When invoked inside a guard, it behaves as if the right-hand side of
+  `when` is injected as part of the guard, replacing the custom guard
+  arguments by the expressions given as inputs.
 
+  When invoked outside of a guard, it preserves regular function calling
+  semantics with one caveat: all arguments are evaluated before invocation,
+  except arguments which are unused, which are then never evaluated.
   """
   @doc since: "1.6.0"
   @spec defguard(Macro.t()) :: Macro.t()

lib/elixir/lib/kernel/utils.ex

@@ -339,7 +339,7 @@ defmodule Kernel.Utils do
           unquote(literal_quote(unquote_every_ref(expr, vars), []))
 
         false ->
-          unquote(literal_quote(unquote_refs_once(expr, vars, env.module), generated: true))
+          unquote(literal_quote(unquote_refs_once(expr, vars, env), generated: true))
       end
     end
   end
@@ -369,7 +369,9 @@ defmodule Kernel.Utils do
   end
 
   # Prefaces `guard` with unquoted versions of `refs`.
-  defp unquote_refs_once(guard, refs, module) do
+  defp unquote_refs_once(guard, refs, %{module: module} = env) do
+    env = %{env | context: nil}
+
     {guard, used_refs} =
       Macro.postwalk(guard, %{}, fn
         {ref, meta, context} = var, acc when is_atom(ref) and is_atom(context) ->
@@ -391,6 +393,12 @@ defmodule Kernel.Utils do
               {var, acc}
           end
 
+        {{:., dot_meta, [:erlang, :orelse]}, meta, [left, right]}, acc ->
+          {Macro.expand({{:., dot_meta, [Kernel, :or]}, meta, [left, right]}, env), acc}
+
+        {{:., dot_meta, [:erlang, :andalso]}, meta, [left, right]}, acc ->
+          {Macro.expand({{:., dot_meta, [Kernel, :and]}, meta, [left, right]}, env), acc}
+
         node, acc ->
           {node, acc}
       end)

lib/elixir/test/elixir/kernel/guard_test.exs

@@ -483,6 +483,44 @@ defmodule Kernel.GuardTest do
              """
     end
 
+    defguard with_or_and_or(foo, bar, baz) when foo or (bar and (baz or foo))
+
+    test "expands conditionals" do
+      args = quote(do: [1 + 1, 2 + 2, 3 + 3])
+
+      assert expand_defguard_to_string(:with_or_and_or, args, :guard) == """
+             :erlang.orelse(1 + 1, :erlang.andalso(2 + 2, :erlang.orelse(3 + 3, 1 + 1)))
+             """
+
+      assert expand_defguard_to_string(:with_or_and_or, args, nil) == """
+             {arg1, arg2, arg3} = {1 + 1, 2 + 2, 3 + 3}
+
+             case arg1 do
+               false ->
+                 case arg2 do
+                   false ->
+                     false
+
+                   true ->
+                     case arg3 do
+                       false -> arg1
+                       true -> true
+                       other -> :erlang.error({:badbool, :or, other})
+                     end
+
+                   other ->
+                     :erlang.error({:badbool, :and, other})
+                 end
+
+               true ->
+                 true
+
+               other ->
+                 :erlang.error({:badbool, :or, other})
+             end
+             """
+    end
+
     defguard in_list(a) when Kernel.in(a, [:test])
 
     test "expands remote functions" do