From 5c64928972c6bed09ae6d44be4070114b595335e Mon Sep 17 00:00:00 2001
From: Lennart Spitzner <hexagoxel@hexagoxel.de>
Date: Tue, 21 Apr 2020 01:34:31 +0200
Subject: [PATCH] Fix problem of do notation as left argument of an operator

---
 src-literatetests/15-regressions.blt          |  7 ++++
 .../Brittany/Internal/Layouters/Expr.hs       | 34 +++++++++++--------
 2 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/src-literatetests/15-regressions.blt b/src-literatetests/15-regressions.blt
index a6a0274..7fa47e0 100644
--- a/src-literatetests/15-regressions.blt
+++ b/src-literatetests/15-regressions.blt
@@ -877,3 +877,10 @@ instance HasDependencies SomeDataModel where
   -- between these data models or whatever.
   type Dependencies SomeDataModel
     = (SomeOtherDataModelId, SomeOtherOtherDataModelId)
+
+#test stupid-do-operator-combination
+
+func =
+  do
+    y
+  >>= x
diff --git a/src/Language/Haskell/Brittany/Internal/Layouters/Expr.hs b/src/Language/Haskell/Brittany/Internal/Layouters/Expr.hs
index bc43fe2..660355c 100644
--- a/src/Language/Haskell/Brittany/Internal/Layouters/Expr.hs
+++ b/src/Language/Haskell/Brittany/Internal/Layouters/Expr.hs
@@ -426,6 +426,9 @@ layoutExpr lexpr@(L _ expr) = do
             (_, L _ (HsApp _ (L _ HsVar{}))) -> False
             _ -> True
 #endif
+      let leftIsDoBlock = case expLeft of
+            L _ HsDo{} -> True
+            _          -> False
       runFilteredAlternative $ do
         -- one-line
         addAlternative
@@ -442,16 +445,17 @@ layoutExpr lexpr@(L _ expr) = do
         --   , docSetBaseY $ docAddBaseY BrIndentRegular expDocRight
         --   ]
         -- two-line
-        addAlternative
-          $ docAddBaseY BrIndentRegular
-          $ docPar
-              expDocLeft -- TODO: this is not forced to single-line, which has
-                         -- certain.. interesting consequences.
-                         -- At least, the "two-line" label is not entirely
-                         -- accurate.
-              ( docForceSingleline
+        addAlternative $ do
+          let
+            expDocOpAndRight = docForceSingleline
               $ docCols ColOpPrefix [appSep $ expDocOp, docSetBaseY expDocRight]
-              )
+          if leftIsDoBlock
+            then docLines [expDocLeft, expDocOpAndRight]
+            else docAddBaseY BrIndentRegular $ docPar expDocLeft expDocOpAndRight
+              -- TODO: in both cases, we don't force expDocLeft to be
+              -- single-line, which has certain.. interesting consequences.
+              -- At least, the "two-line" label is not entirely
+              -- accurate.
         -- one-line + par
         addAlternativeCond allowPar
           $ docSeq
@@ -460,11 +464,13 @@ layoutExpr lexpr@(L _ expr) = do
           , docForceParSpacing expDocRight
           ]
         -- more lines
-        addAlternative
-          $ docAddBaseY BrIndentRegular
-          $ docPar
-              expDocLeft
-              (docCols ColOpPrefix [appSep expDocOp, docSetBaseY expDocRight])
+        addAlternative $ do
+          let expDocOpAndRight =
+                docCols ColOpPrefix [appSep expDocOp, docSetBaseY expDocRight]
+          if leftIsDoBlock
+            then docLines [expDocLeft, expDocOpAndRight]
+            else docAddBaseY BrIndentRegular
+                   $ docPar expDocLeft expDocOpAndRight
 #if MIN_VERSION_ghc(8,6,0)   /* ghc-8.6 */
     NegApp _ op _ -> do
 #else