if: treat is_nil as a negator

Author: novaugust

CHANGELOG.md

@@ -8,6 +8,13 @@ they can and will change without that change being reflected in Styler's semanti
 ### Improvements
 
 - `if`: drop empty `do` bodies like `if a, do: nil, else: b` => `if !a, do: b` (#227)
+- `if`: treat `is_nil` as a negator.
+
+  this means `if is_nil(x), do: a, else: b` will be inverted to `if x, do: b, else: a`
+
+  or `if !is_nil(x), do: y` will be rewritten as `if x, do: y`
+
+  This could cause problems where x is `false` =)
 
 ### Fixes
 

docs/control_flow_macros.md

@@ -69,7 +69,7 @@ if !a, do: b
 ### Negation Inversion
 
 Styler removes negators in the head of `if` statements by "inverting" the statement.
-The following operators are considered "negators": `!`, `not`, `!=`, `!==`
+The following operators are considered "negators": `!`, `not`, `!=`, `!==`, `is_nil`
 
 Examples:
 
@@ -83,7 +83,12 @@ if x, do: z, else: y
 Because elixir relies on truthy/falsey values for its `if` statements, boolean casting is unnecessary and so double negation is simply removed.
 
 ```elixir
-if !!x, do: y
+if !x, do: y
+# styled:
+if x, do: y
+
+# similarly
+if !is_nil(x), do: y
 # styled:
 if x, do: y
 ```

lib/style/blocks.ex

@@ -30,7 +30,7 @@ defmodule Styler.Style.Blocks do
   alias Styler.Style
   alias Styler.Zipper
 
-  defguardp is_negator(n) when elem(n, 0) in [:!, :not, :!=, :!==]
+  defguardp is_if_negator(n) when elem(n, 0) in [:!, :not, :!=, :!==, :is_nil]
   defguardp is_empty_body(n) when elem(n, 0) == :__block__ and elem(n, 2) in [[nil], []]
 
   # case statement with exactly 2 `->` cases
@@ -132,12 +132,12 @@ defmodule Styler.Style.Blocks do
     case children do
       # double negator
       # if !!x, do: y[, else: ...] => if x, do: y[, else: ...]
-      [{_, _, [nb]} = na, do_else] when is_negator(na) and is_negator(nb) ->
+      [{_, _, [nb]} = na, do_else] when is_if_negator(na) and is_if_negator(nb) ->
         zipper |> Zipper.replace({:if, m, [invert(nb), do_else]}) |> run(ctx)
 
       # Credo.Check.Refactor.NegatedConditionsWithElse
       # if !x, do: y, else: z => if x, do: z, else: y
-      [negator, [{do_, do_body}, {else_, else_body}]] when is_negator(negator) ->
+      [negator, [{do_, do_body}, {else_, else_body}]] when is_if_negator(negator) ->
         zipper |> Zipper.replace({:if, m, [invert(negator), [{do_, else_body}, {else_, do_body}]]}) |> run(ctx)
 
       # drop `else end` and `else: nil`
@@ -368,5 +368,6 @@ defmodule Styler.Style.Blocks do
   defp invert({:!, _, [condition]}), do: condition
   defp invert({:not, _, [condition]}), do: condition
   defp invert({:in, m, [_, _]} = ast), do: {:not, m, [ast]}
+  defp invert({:is_nil, _, [a]}), do: a
   defp invert({_, m, _} = ast), do: {:!, [line: m[:line]], [ast]}
 end

test/style/blocks_test.exs

@@ -1012,6 +1012,12 @@ defmodule Styler.Style.BlocksTest do
         end
         """
       )
+
+      assert_style "if is_nil(a.b), do: nil, else: a.c", "if a.b, do: a.c"
+    end
+
+    test "if not is_nil" do
+      assert_style "if is_nil(a), do: b, else: c", "if a, do: c, else: b"
     end
 
     test "double negator rewrites" do