From 47918282bb3a87041f28e6a419e311c2c76dcad0 Mon Sep 17 00:00:00 2001
From: David Feuer <David.Feuer@gmail.com>
Date: Mon, 9 May 2022 16:21:15 -0400
Subject: [PATCH] [#265] Make SuperComposition less brittle

* Use overlapping instances instead of incoherent ones. Fixes #265.

* Make the first argument of `...` a function unconditionally, before
  instance selection. This can theoretically improve inference slightly,
  though it probably doesn't have much impact in practice.
---
 CHANGES.md              |  4 ++++
 benchmark/Main.hs       |  2 +-
 src/Universum/VarArg.hs | 12 ++++++------
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index ece3451..3b4148c 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,10 @@
 Unreleased
 =====
 
+* [#265](https://github.com/serokell/universum/issues/265):
+  Make `SuperComposition` inference less brittle, and give it four
+  type parameters.
+
 * [#252](https://github.com/serokell/universum/pull/252):
   Remove `Option` re-export. Use `Maybe` instead.
 
diff --git a/benchmark/Main.hs b/benchmark/Main.hs
index b507209..71703a2 100644
--- a/benchmark/Main.hs
+++ b/benchmark/Main.hs
@@ -112,7 +112,7 @@ bgroupSuperComposition = bgroup "(...)"
  where
   super10 :: [()] -> Bool
   super10 = null
-        ... ((: []) ... Unsafe.head ... pure ... Unsafe.head
+        ... ((: []) ... Unsafe.head ... (:[]) ... Unsafe.head
         ... (: [(), (), (), ()]) ... Unsafe.head ... (: []) ... Unsafe.head
         ... (: [()]) ... Unsafe.head ... (: [(), ()]) ... Unsafe.head :: [()] -> [()])
 
diff --git a/src/Universum/VarArg.hs b/src/Universum/VarArg.hs
index 18f7cae..de0ee80 100644
--- a/src/Universum/VarArg.hs
+++ b/src/Universum/VarArg.hs
@@ -19,7 +19,7 @@ module Universum.VarArg
 -- >>> import Data.List (zip5)
 
 -- | This type class allows to implement variadic composition operator.
-class SuperComposition a b c | a b -> c where
+class SuperComposition x y b r | x y b -> r where
     -- | Allows to apply function to result of another function with multiple
     -- arguments.
     --
@@ -45,16 +45,16 @@ class SuperComposition a b c | a b -> c where
     -- disappear due to very general inferred type. However, functions without type
     -- specification but with applied @INLINE@ pragma are fast again.
     --
-    (...) :: a -> b -> c
+    (...) :: (x -> y) -> b -> r
 
 infixl 8 ...
 
-instance {-# INCOHERENT #-} (a ~ c, r ~ b) =>
-         SuperComposition (a -> b) c r where
+instance {-# OVERLAPPABLE #-} (x ~ b, y ~ r) =>
+         SuperComposition x y b r where
     f ... g = f g
     {-# INLINE (...) #-}
 
-instance {-# INCOHERENT #-} (SuperComposition (a -> b) d r1, r ~ (c -> r1)) =>
-         SuperComposition (a -> b) (c -> d) r where
+instance {-# OVERLAPPING #-} (SuperComposition x y d r1, r ~ (c -> r1)) =>
+         SuperComposition x y (c -> d) r where
     (f ... g) c = f ... g c
     {-# INLINE (...) #-}