Clean up "Eta reduction for data families" Notes

Before, there were two distinct Notes named
"Eta reduction for data families". This renames one of them to
"Implementing eta reduction for data families" to disambiguate the
two and fixes references in other parts of the codebase to ensure
that they are pointing to the right place.

Fixes #17313.

[ci skip]

1 files changed, 8 insertions, 6 deletions

diff --git a/compiler/GHC/Core/FamInstEnv.hs b/compiler/GHC/Core/FamInstEnv.hs
index 8ac78035bd..0b58195be6 100644
--- a/compiler/GHC/Core/FamInstEnv.hs
+++ b/compiler/GHC/Core/FamInstEnv.hs
@@ -118,6 +118,7 @@ data FamInst  -- See Note [FamInsts and CoAxioms]
 
             , fi_tys    :: [Type]       --   The LHS type patterns
             -- May be eta-reduced; see Note [Eta reduction for data families]
+            -- in GHC.Core.Coercion.Axiom
 
             , fi_rhs :: Type         --   the RHS, with its freshened vars
             }
@@ -132,7 +133,8 @@ Note [Arity of data families]
 Data family instances might legitimately be over- or under-saturated.
 
 Under-saturation has two potential causes:
- U1) Eta reduction. See Note [Eta reduction for data families].
+ U1) Eta reduction. See Note [Eta reduction for data families] in
+     GHC.Core.Coercion.Axiom.
  U2) When the user has specified a return kind instead of written out patterns.
      Example:
 
@@ -160,8 +162,8 @@ Over-saturation is also possible:
 
       However, we require that any over-saturation is eta-reducible. That is,
       we require that any extra patterns be bare unrepeated type variables;
-      see Note [Eta reduction for data families]. Accordingly, the FamInst
-      is never over-saturated.
+      see Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom.
+      Accordingly, the FamInst is never over-saturated.
 
 Why can we allow such flexibility for data families but not for type families?
 Because data families can be decomposed -- that is, they are generative and
@@ -335,7 +337,7 @@ Then we get a data type for each instance, and an axiom:
    axiom ax8 a :: T Bool [a] ~ TBoolList a
 
 These two axioms for T, one with one pattern, one with two;
-see Note [Eta reduction for data families]
+see Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom
 
 Note [FamInstEnv determinism]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -479,7 +481,7 @@ irrelevant (clause 1 of compatible) or benign (clause 2 of compatible).
 Note [Compatibility of eta-reduced axioms]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In newtype instances of data families we eta-reduce the axioms,
-See Note [Eta reduction for data families] in GHC.Core.FamInstEnv. This means that
+See Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom. This means that
 we sometimes need to test compatibility of two axioms that were eta-reduced to
 different degrees, e.g.:
 
@@ -1057,7 +1059,7 @@ We handle data families and type families separately here:
  * For data family instances, though, we need to re-split for each
    instance, because the breakdown might be different for each
    instance.  Why?  Because of eta reduction; see
-   Note [Eta reduction for data families].
+   Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom.
 -}
 
 -- checks if one LHS is dominated by a list of other branches