[Git][ghc/ghc][wip/spj-add-nospec-notes] Clarify a couple of Notes about 'nospec'

[Simon Peyton Jones (@simonpj)]

Simon Peyton Jones pushed to branch wip/spj-add-nospec-notes at Glasgow Haskell Compiler / GHC


Commits:
843eda05 by Simon Peyton Jones at 2023-04-11T13:10:49+01:00
Clarify a couple of Notes about 'nospec'

- - - - -


2 changed files:

- compiler/GHC/Core/InstEnv.hs
- compiler/GHC/Types/Id/Make.hs


Changes:

=====================================
compiler/GHC/Core/InstEnv.hs
=====================================
@@ -849,8 +849,10 @@ Here are the moving parts:
 
 * `GHC.HsToCore.Binds.dsHsWrapper` desugars the evidence application (f d) into
   (nospec f d) if `d` is incoherent. It has to do a dependency analysis to
-  determine transitive dependencies, but we need to do that anway.
+  determine transitive dependencies, but we need to do that anyway.
   See Note [Desugaring incoherent evidence] in GHC.HsToCore.Binds.
+
+  See also Note [nospecId magic] in GHC.Types.Id.Make.
 -}
 
 type DFunInstType = Maybe Type


=====================================
compiler/GHC/Types/Id/Make.hs
=====================================
@@ -2002,10 +2002,15 @@ Note that this happens *after* unfoldings are exposed in the interface file.
 This is crucial: otherwise, we could import an unfolding in which
 'nospec' has been inlined (= erased), and we would lose the benefit.
 
-'nospec' is used in the implementation of 'withDict': we insert 'nospec'
-so that the typeclass specialiser doesn't assume any two evidence terms
-of the same type are equal. See Note [withDict] in GHC.Tc.Instance.Class,
-and see test case T21575b for an example.
+'nospec' is used:
+
+* In the implementation of 'withDict': we insert 'nospec' so that the
+  typeclass specialiser doesn't assume any two evidence terms of the
+  same type are equal. See Note [withDict] in GHC.Tc.Instance.Class,
+  and see test case T21575b for an example.
+
+* To defeat the specialiser when we have incoherent instances.
+  See Note [Coherence and specialisation: overview] in GHC.Core.InstEnv.
 
 Note [The oneShot function]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/843eda05bc82dee0882750542d866dee52160c93

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/843eda05bc82dee0882750542d866dee52160c93
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