monoid-extras 0.7 → 0.7.0.1

raw patch · 4 files changed

+33/−7 lines, 4 files

Files

• CHANGES +6/−0
• monoid-extras.cabal +1/−1
• src/Data/Monoid/Action.hs +14/−6
• src/Data/Monoid/SemiDirectProduct.hs +12/−0

@@ -1,3 +1,9 @@+* 0.7.0.1: 29 January 2026++  - Add some cautionary comments to `Regular` and `Semi`.  In+    particular, `Regular` cannot be used to create a lawful semidirect+    product of a monoid/semigroup with itself. ([#63](https://github.com/diagrams/monoid-extras/issues/63))+ * 0.7: 12 May 2025    - Updates to `Data.Monoid.Coproduct`:

@@ -1,5 +1,5 @@ name:                monoid-extras-version:             0.7+version:             0.7.0.1 synopsis:            Various extra monoid-related definitions and utilities description:         Various extra monoid-related definitions and utilities,                      such as monoid actions, monoid coproducts, semi-direct

@@ -39,12 +39,16 @@ --   * @act (m1 \`mappend\` m2) = act m1 . act m2@ -- --   Semigroup instances are required to satisfy the second law but with---   ('<>') instead of 'mappend'.  Additionally, if the type @s@ has---   any algebraic structure, @act m@ should be a homomorphism.  For---   example, if @s@ is also a monoid we should have @act m mempty =---   mempty@ and @act m (s1 \`mappend\` s2) = (act m s1) \`mappend\`---   (act m s2)@.+--   ('<>') instead of 'mappend'. --+--   Additionally, if the type @s@ has any algebraic structure, @act+--   m@ should typically be a homomorphism.  For example, if @s@ is+--   also a monoid we should have @act m mempty = mempty@ and @act m+--   (s1 \`mappend\` s2) = (act m s1) \`mappend\` (act m s2)@.  In+--   particular, these laws are necessary for the semidirect product+--   @Semi s m@ to be a valid semigroup/monoid.  For a more+--   fine-grained treatment of these ideas, see the @lr-acts@ package.+-- --   By default, @act = const id@, so for a type @M@ which should have --   no action on anything, it suffices to write --@@ -123,7 +127,11 @@ -- | Any monoid acts on itself by left multiplication. --   This newtype witnesses this action: --   @'getRegular' $ 'Regular' m1 `'act'` 'Regular' m2 = m1 '<>' m2@-newtype Regular m = Regular { getRegular :: m }+--+--   Note that this typically does NOT satisfy the distributivity law+--   @m `act` (m1 <> m2) = (m `act` m1) <> (m `act` m2)@, and hence+--   cannot be used to form a lawful semidirect product of a monoid with itself.+newtype Regular m = Regular {getRegular :: m}  instance Semigroup m => Action m (Regular m) where   m1 `act` Regular m2 = Regular $ m1 <> m2

@@ -24,6 +24,18 @@ --   We think of the @m@ values as a "tag" decorating the @s@ values, --   which also affect the way the @s@ values combine. --+--   NOTE: this is only a valid semigroup/monoid if the action of @m@+--   on @s@ satisfies BOTH:+--+--     1. @act@ is a monoid/semigroup homomorphism from @m@ to @(s ->+--       s)@, that is, @act mempty = id@ and @act (m1 <> m2) = act m1+--       . act m2@+--     2. @act m@ is a monoid/semigroup homomorphism for any @m@, that is,+--       @act m mempty = mempty@ and @act m (s1 <> s2) = act m s1 <> act m s2@.+--+--   For a more fine-grained treatment of these ideas, see the+--   @lr-acts@ package.+-- --   We call the monoid @m@ the quotient monoid and the monoid @s@ the --   sub-monoid of the semi-direct product. The semi-direct product --   @Semi s m@ is an extension of the monoid @s@ with @m@ being the