[Git][ghc/ghc][master] base: Define Semigroup and Monoid instances for lazy ST

15 Dec 2025


      Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
c32de3b0 by Johan Förberg at 2025-12-15T02:36:03-05:00
base: Define Semigroup and Monoid instances for lazy ST

CLC proposal:
https://github.com/haskell/core-libraries-committee/issues/374

Fixes #26581

- - - - -


6 changed files:

- libraries/base/changelog.md
- libraries/ghc-internal/src/GHC/Internal/Control/Monad/ST/Lazy/Imp.hs
- testsuite/tests/interface-stability/base-exports.stdout
- testsuite/tests/interface-stability/base-exports.stdout-javascript-unknown-ghcjs
- testsuite/tests/interface-stability/base-exports.stdout-mingw32
- testsuite/tests/interface-stability/base-exports.stdout-ws-32


Changes:

=====================================
libraries/base/changelog.md
=====================================
@@ -14,6 +14,7 @@
   * Remove extra laziness from `Data.Bifunctor.Bifunctor` instances for all tuples to have the same laziness as their `Data.Functor.Functor` counterparts (i.e. they became more strict than before) ([CLC proposal #339](https://github.com/haskell/core-libraries-committee/issues/339))
   * Adjust the strictness of `Data.List.iterate'` to be more reasonable: every element of the output list is forced to WHNF when the `(:)` containing it is forced. ([CLC proposal #335)](https://github.com/haskell/core-libraries-committee/issues/335)
   * Add `nubOrd` / `nubOrdBy` to `Data.List` and `Data.List.NonEmpty`. ([CLC proposal #336](https://github.com/haskell/core-libraries-committee/issues/336))
+  * Add `Semigroup` and `Monoid` instances for `Control.Monad.ST.Lazy`. ([CLC proposal #374](https://github.com/haskell/core-libraries-committee/issues/374))
 
 ## 4.22.0.0 *TBA*
   * Shipped with GHC 9.14.1


=====================================
libraries/ghc-internal/src/GHC/Internal/Control/Monad/ST/Lazy/Imp.hs
=====================================
@@ -214,6 +214,14 @@ fixST m = ST (\ s ->
 instance MonadFix (ST s) where
         mfix = fixST
 
+-- | @since base-4.23.0.0
+instance Semigroup a => Semigroup (ST s a) where
+    (<>) = liftA2 (<>)
+
+-- | @since base-4.23.0.0
+instance Monoid a => Monoid (ST s a) where
+    mempty = pure mempty
+
 -- ---------------------------------------------------------------------------
 -- Strict <--> Lazy
 


=====================================
testsuite/tests/interface-stability/base-exports.stdout
=====================================
@@ -11318,6 +11318,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Monoid a => GHC.Internal.Base.Mo
 instance forall a. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Monoid GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Monoid Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’
@@ -11372,6 +11373,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Semigroup a => GHC.Internal.Base
 instance forall a. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Semigroup GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Semigroup Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.Bits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’


=====================================
testsuite/tests/interface-stability/base-exports.stdout-javascript-unknown-ghcjs
=====================================
@@ -14364,6 +14364,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Monoid a => GHC.Internal.Base.Mo
 instance forall a. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Monoid GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Monoid Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’
@@ -14415,6 +14416,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Semigroup a => GHC.Internal.Base
 instance forall a. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Semigroup GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Semigroup Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.Bits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’


=====================================
testsuite/tests/interface-stability/base-exports.stdout-mingw32
=====================================
@@ -11580,6 +11580,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Monoid a => GHC.Internal.Base.Mo
 instance forall a. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Monoid GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Monoid Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’
@@ -11632,6 +11633,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Semigroup a => GHC.Internal.Base
 instance forall a. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Semigroup GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Semigroup Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.Bits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’


=====================================
testsuite/tests/interface-stability/base-exports.stdout-ws-32
=====================================
@@ -11318,6 +11318,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Monoid a => GHC.Internal.Base.Mo
 instance forall a. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Monoid GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Monoid a => GHC.Internal.Base.Monoid (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Monoid Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Monoid (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’
@@ -11372,6 +11373,7 @@ instance forall a k (b :: k). GHC.Internal.Base.Semigroup a => GHC.Internal.Base
 instance forall a. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Conc.Sync.STM a) -- Defined in ‘GHC.Internal.Conc.Sync’
 instance GHC.Internal.Base.Semigroup GHC.Internal.Exception.Context.ExceptionContext -- Defined in ‘GHC.Internal.Exception.Context’
 instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.ST.ST s a) -- Defined in ‘GHC.Internal.ST’
+instance forall a s. GHC.Internal.Base.Semigroup a => GHC.Internal.Base.Semigroup (GHC.Internal.Control.Monad.ST.Lazy.Imp.ST s a) -- Defined in ‘GHC.Internal.Control.Monad.ST.Lazy.Imp’
 instance GHC.Internal.Base.Semigroup Data.Array.Byte.ByteArray -- Defined in ‘Data.Array.Byte’
 instance forall a. GHC.Internal.Bits.Bits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.And a) -- Defined in ‘GHC.Internal.Data.Bits’
 instance forall a. GHC.Internal.Bits.FiniteBits a => GHC.Internal.Base.Semigroup (GHC.Internal.Data.Bits.Iff a) -- Defined in ‘GHC.Internal.Data.Bits’



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c32de3b005034f3c17c849d395f46a63...

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