Remove Expression.If constructor

We can just use Case and pretty-print it as an if expression when it's
on the appropriate form.

Author: ollef

src/Language/Elm/Expression.hs

@@ -30,7 +30,6 @@ data Expression v
   | Record [(Name.Field, Expression v)]
   | Proj Name.Field
   | Case (Expression v) [(Pattern Int, Scope Int Expression v)]
-  | If (Expression v) (Expression v) (Expression v)
   | List [Expression v]
   | String !Text
   | Int !Integer
@@ -50,7 +49,6 @@ instance Monad Expression where
   Record fs >>= f = Record [(fname, e >>= f) | (fname, e) <- fs]
   Proj f >>= _ = Proj f
   Case e brs >>= f = Case (e >>= f) [(pat, s >>>= f) | (pat, s) <- brs]
-  If e e1 e2 >>= f = If (e >>= f) (e1 >>= f) (e2 >>= f)
   List es >>= f = List ((>>= f) <$> es)
   String s >>= _ = String s
   Int n >>= _ = Int n
@@ -81,6 +79,13 @@ appsView = go mempty
         _ ->
           (expr, args)
 
+if_ :: Expression v -> Expression v -> Expression v -> Expression v
+if_ bool_ true false =
+  Case bool_
+    [ (Pattern.Con "Basics.True" [], Scope $ pure $ pure true)
+    , (Pattern.Con "Basics.False" [], Scope $ pure $ pure false)
+    ]
+
 (|>) :: Expression v -> Expression v -> Expression v
 (|>) e1 e2 = apps "Basics.|>" [e1, e2]
 
@@ -133,9 +138,6 @@ foldMapGlobals f expr =
         (bifoldMap (Pattern.foldMapGlobals f) (foldMapGlobals f . Bound.fromScope))
         branches
 
-    If e e1 e2 ->
-      foldMapGlobals f e <> foldMapGlobals f e1 <> foldMapGlobals f e2
-
     List es ->
       foldMap (foldMapGlobals f) es
 

src/Language/Elm/Pretty.hs

@@ -457,6 +457,17 @@ expression env prec expr =
     Expression.Proj f ->
       "." <> field f
 
+    Expression.Case bool_
+      [ (Pattern.Con "Basics.True" [], unusedScope -> Just true)
+      , (Pattern.Con "Basics.False" [], unusedScope -> Just false)
+      ] ->
+      parensWhen (prec > ifPrec) $
+        "if" <+> expression env 0 bool_ <+> "then" <> line <>
+          indent 4 (expression env 0 true) <> line <>
+        line <>
+        "else" <> line <>
+          indent 4 (expression env 0 false)
+
     Expression.Case expr' branches ->
       parensWhen (prec > casePrec) $
         "case" <+> expression env 0 expr' <+> "of" <> line <>
@@ -472,14 +483,6 @@ expression env prec expr =
           ]
         )
 
-    Expression.If expr' true false ->
-      parensWhen (prec > ifPrec) $
-        "if" <+> expression env 0 expr' <+> "then" <> line <>
-          indent 4 (expression env 0 true) <> line <>
-        line <>
-        "else" <> line <>
-          indent 4 (expression env 0 false)
-
     Expression.List exprs ->
       list $ expression env 0 <$> exprs
 
@@ -663,3 +666,7 @@ casePrec = 0
 ifPrec = 0
 funPrec = 0
 projPrec = 11
+
+unusedScope :: (Monad f, Traversable f) => Bound.Scope b f a -> Maybe (f a)
+unusedScope =
+  traverse (Bound.unvar (const Nothing) pure) . Bound.fromScope