When outputting list of available instances, sort it.

Summary:
The intent of this commit is to make test suite cases more stable, so that
it doesn't matter what order we load interface files in, the test output
doesn't change.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D484

compiler/typecheck/TcErrors.lhs

@@ -44,7 +44,7 @@ import StaticFlags      ( opt_PprStyle_Debug )
 import ListSetOps       ( equivClasses )
 
 import Data.Maybe
-import Data.List        ( partition, mapAccumL, zip4, nub )
+import Data.List        ( partition, mapAccumL, zip4, nub, sortBy )
 \end{code}
 
 %************************************************************************
@@ -1062,7 +1062,7 @@ mk_dict_err fam_envs ctxt (ct, (matches, unifiers, safe_haskell))
         hang (if isSingleton unifiers
               then ptext (sLit "Note: there is a potential instance available:")
               else ptext (sLit "Note: there are several potential instances:"))
-           2 (ppr_insts unifiers)
+           2 (ppr_insts (sortBy fuzzyClsInstCmp unifiers))
 
     -- Report "potential instances" only when the constraint arises
     -- directly from the user's use of an overloaded function

compiler/types/InstEnv.lhs

@@ -15,6 +15,7 @@ module InstEnv (
         ClsInst(..), DFunInstType, pprInstance, pprInstanceHdr, pprInstances,
         instanceHead, instanceSig, mkLocalInstance, mkImportedInstance,
         instanceDFunId, tidyClsInstDFun, instanceRoughTcs,
+        fuzzyClsInstCmp,
 
         InstEnv, emptyInstEnv, extendInstEnv, deleteFromInstEnv, identicalInstHead, 
         extendInstEnvList, lookupUniqueInstEnv, lookupInstEnv', lookupInstEnv, instEnvElts,
@@ -42,6 +43,9 @@ import Id
 import FastString
 import Data.Data        ( Data, Typeable )
 import Data.Maybe       ( isJust, isNothing )
+#if __GLASGOW_HASKELL__ < 709
+import Data.Monoid
+#endif
 \end{code}
 
 
@@ -76,6 +80,18 @@ data ClsInst
                                         -- the decl of BasicTypes.OverlapFlag
     }
   deriving (Data, Typeable)
+
+-- | A fuzzy comparison function for class instances, intended for sorting
+-- instances before displaying them to the user.
+fuzzyClsInstCmp :: ClsInst -> ClsInst -> Ordering
+fuzzyClsInstCmp x y =
+    stableNameCmp (is_cls_nm x) (is_cls_nm y) `mappend`
+    mconcat (map cmp (zip (is_tcs x) (is_tcs y)))
+  where
+    cmp (Nothing, Nothing) = EQ
+    cmp (Nothing, Just _) = LT
+    cmp (Just _, Nothing) = GT
+    cmp (Just x, Just y) = stableNameCmp x y
 \end{code}
 
 Note [Template tyvars are fresh]

testsuite/tests/annotations/should_fail/annfail10.stderr

@@ -3,12 +3,14 @@ annfail10.hs:9:1:
     No instance for (Data.Data.Data a0) arising from an annotation
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
-      instance Data.Data.Data () -- Defined in ‘Data.Data’
       instance (Data.Data.Data a, Data.Data.Data b) =>
-               Data.Data.Data (a, b)
+               Data.Data.Data (Either a b)
         -- Defined in ‘Data.Data’
-      instance (Data.Data.Data a, Data.Data.Data b, Data.Data.Data c) =>
-               Data.Data.Data (a, b, c)
+      instance Data.Data.Data t => Data.Data.Data (Data.Proxy.Proxy t)
+        -- Defined in ‘Data.Data’
+      instance (GHC.Types.Coercible a b, Data.Data.Data a,
+                Data.Data.Data b) =>
+               Data.Data.Data (Data.Type.Coercion.Coercion a b)
         -- Defined in ‘Data.Data’
       ...plus 31 others
     In the annotation: {-# ANN f 1 #-}
@@ -17,8 +19,8 @@ annfail10.hs:9:11:
     No instance for (Num a0) arising from the literal ‘1’
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
-      instance Num Double -- Defined in ‘GHC.Float’
-      instance Num Float -- Defined in ‘GHC.Float’
-      instance Num Int -- Defined in ‘GHC.Num’
+      instance Num GHC.Int.Int16 -- Defined in ‘GHC.Int’
+      instance Num GHC.Int.Int32 -- Defined in ‘GHC.Int’
+      instance Num GHC.Int.Int64 -- Defined in ‘GHC.Int’
       ...plus 11 others
     In the annotation: {-# ANN f 1 #-}

testsuite/tests/ghci.debugger/scripts/break006.stderr

@@ -5,9 +5,9 @@
     Use :print or :force to determine these types
     Relevant bindings include it :: t1 (bound at <interactive>:6:1)
     Note: there are several potential instances:
-      instance Show Double -- Defined in ‘GHC.Float’
-      instance Show Float -- Defined in ‘GHC.Float’
-      instance Show () -- Defined in ‘GHC.Show’
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
+      instance Show Ordering -- Defined in ‘GHC.Show’
+      instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 22 others
     In a stmt of an interactive GHCi command: print it
 
@@ -17,8 +17,8 @@
     Use :print or :force to determine these types
     Relevant bindings include it :: t1 (bound at <interactive>:8:1)
     Note: there are several potential instances:
-      instance Show Double -- Defined in ‘GHC.Float’
-      instance Show Float -- Defined in ‘GHC.Float’
-      instance Show () -- Defined in ‘GHC.Show’
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
+      instance Show Ordering -- Defined in ‘GHC.Show’
+      instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 22 others
     In a stmt of an interactive GHCi command: print it

testsuite/tests/ghci.debugger/scripts/print019.stderr

@@ -5,8 +5,8 @@
     Use :print or :force to determine these types
     Relevant bindings include it :: a1 (bound at <interactive>:11:1)
     Note: there are several potential instances:
-      instance Show Unary -- Defined at ../Test.hs:37:29
-      instance Show a => Show (MkT2 a) -- Defined at ../Test.hs:20:12
-      instance Show a => Show (MkT a) -- Defined at ../Test.hs:17:13
+      instance Show TyCon -- Defined in ‘Data.Typeable.Internal’
+      instance Show TypeRep -- Defined in ‘Data.Typeable.Internal’
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
       ...plus 30 others
     In a stmt of an interactive GHCi command: print it

testsuite/tests/ghci/scripts/Defer02.stderr

@@ -69,9 +69,9 @@
     No instance for (Num a1) arising from the literal ‘23’
     The type variable ‘a1’ is ambiguous
     Note: there are several potential instances:
+      instance Num Integer -- Defined in ‘GHC.Num’
       instance Num Double -- Defined in ‘GHC.Float’
       instance Num Float -- Defined in ‘GHC.Float’
-      instance Num Int -- Defined in ‘GHC.Num’
       ...plus two others
     In the first argument of ‘myOp’, namely ‘23’
     In the expression: myOp 23

testsuite/tests/overloadedlists/should_fail/overloadedlistsfail01.stderr

@@ -3,9 +3,9 @@ overloadedlistsfail01.hs:5:8:
     No instance for (Show a0) arising from a use of ‘print’
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
-      instance Show Double -- Defined in ‘GHC.Float’
-      instance Show Float -- Defined in ‘GHC.Float’
-      instance Show () -- Defined in ‘GHC.Show’
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
+      instance Show Ordering -- Defined in ‘GHC.Show’
+      instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 22 others
     In the expression: print [1]
     In an equation for ‘main’: main = print [1]
@@ -25,9 +25,9 @@ overloadedlistsfail01.hs:5:15:
       arising from the literal ‘1’
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
+      instance Num Integer -- Defined in ‘GHC.Num’
       instance Num Double -- Defined in ‘GHC.Float’
       instance Num Float -- Defined in ‘GHC.Float’
-      instance Num Int -- Defined in ‘GHC.Num’
       ...plus two others
     In the expression: 1
     In the first argument of ‘print’, namely ‘[1]’

testsuite/tests/typecheck/should_compile/holes2.stderr

@@ -3,9 +3,9 @@ holes2.hs:3:5: Warning:
     No instance for (Show a0) arising from a use of ‘show’
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
-      instance Show Double -- Defined in ‘GHC.Float’
-      instance Show Float -- Defined in ‘GHC.Float’
-      instance Show () -- Defined in ‘GHC.Show’
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
+      instance Show Ordering -- Defined in ‘GHC.Show’
+      instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 22 others
     In the expression: show _
     In an equation for ‘f’: f = show _

testsuite/tests/typecheck/should_fail/T7857.stderr

@@ -6,10 +6,10 @@ T7857.hs:8:11:
       at T7857.hs:8:1-21
     The type variable ‘r0’ is ambiguous
     Note: there are several potential instances:
-      instance [safe] (PrintfArg a, PrintfType r) => PrintfType (a -> r)
-        -- Defined in ‘Text.Printf’
       instance [safe] (a ~ ()) => PrintfType (IO a)
         -- Defined in ‘Text.Printf’
+      instance [safe] (PrintfArg a, PrintfType r) => PrintfType (a -> r)
+        -- Defined in ‘Text.Printf’
       instance [safe] IsChar c => PrintfType [c]
         -- Defined in ‘Text.Printf’
     In the second argument of ‘($)’, namely ‘printf "" i’

testsuite/tests/typecheck/should_fail/tcfail008.stderr

@@ -4,9 +4,9 @@ tcfail008.hs:3:5:
     The type variable ‘a0’ is ambiguous
     Relevant bindings include o :: [a0] (bound at tcfail008.hs:3:1)
     Note: there are several potential instances:
+      instance Num Integer -- Defined in ‘GHC.Num’
       instance Num Double -- Defined in ‘GHC.Float’
       instance Num Float -- Defined in ‘GHC.Float’
-      instance Num Int -- Defined in ‘GHC.Num’
       ...plus two others
     In the first argument of ‘(:)’, namely ‘1’
     In the expression: 1 : 2

testsuite/tests/typecheck/should_fail/tcfail043.stderr

@@ -29,8 +29,8 @@ tcfail043.hs:40:25:
       a :: a0 (bound at tcfail043.hs:38:6)
       search :: a0 -> [a0] -> Bool (bound at tcfail043.hs:37:1)
     Note: there are several potential instances:
-      instance Eq_ a => Eq_ [a] -- Defined at tcfail043.hs:23:10
       instance Eq_ Int -- Defined at tcfail043.hs:20:10
+      instance Eq_ a => Eq_ [a] -- Defined at tcfail043.hs:23:10
     In the expression: eq a (hd bs)
     In the expression: if eq a (hd bs) then True else search a (tl bs)
     In the expression:

testsuite/tests/typecheck/should_fail/tcfail072.stderr

@@ -8,8 +8,9 @@ tcfail072.hs:23:13:
     The type variable ‘p0’ is ambiguous
     Note: there are several potential instances:
       instance Ord a => Ord (Maybe a) -- Defined in ‘GHC.Base’
-      instance Ord () -- Defined in ‘GHC.Classes’
-      instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
+      instance Ord Ordering -- Defined in ‘GHC.Classes’
+      instance Ord integer-gmp-1.0.0.0:GHC.Integer.Type.BigNat
+        -- Defined in ‘integer-gmp-1.0.0.0:GHC.Integer.Type’
       ...plus 23 others
     In the expression: g A
     In an equation for ‘g’: g (B _ _) = g A

testsuite/tests/typecheck/should_fail/tcfail133.stderr

@@ -6,10 +6,9 @@ tcfail133.hs:68:7:
     No instance for (Show r0) arising from a use of ‘show’
     The type variable ‘r0’ is ambiguous
     Note: there are several potential instances:
-      instance Show Zero -- Defined at tcfail133.hs:8:29
-      instance Show One -- Defined at tcfail133.hs:9:28
-      instance (Show a, Show b, Number a, Digit b) => Show (a :@ b)
-        -- Defined at tcfail133.hs:11:54
+      instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
+      instance Show Ordering -- Defined in ‘GHC.Show’
+      instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 25 others
     In the expression: show
     In the expression: show $ add (One :@ Zero) (One :@ One)

testsuite/tests/typecheck/should_fail/tcfail181.stderr

@@ -7,9 +7,9 @@ tcfail181.hs:17:9:
       at tcfail181.hs:17:1-30
     The type variable ‘m0’ is ambiguous
     Note: there are several potential instances:
-      instance Monad ((->) r) -- Defined in ‘GHC.Base’
-      instance Monad IO -- Defined in ‘GHC.Base’
       instance Monad Maybe -- Defined in ‘GHC.Base’
+      instance Monad IO -- Defined in ‘GHC.Base’
+      instance Monad ((->) r) -- Defined in ‘GHC.Base’
       ...plus one other
     In the expression: foo
     In the expression: foo {bar = return True}