Add composition of MonadTransformers
Nobody has claimed this yet.
Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 45/100
Research direction
Start by reading the proposed ComposeT definition and the existing MonadTrans implementations in the transformers package. Check how the type and kind signatures fit the library's supported language features, then verify that the OkT deriving example works; done means composed transformers can provide MonadTrans as described.
Written by the indexing model from the issue text.
Description
[original issue 84 by @Icelandjack]
Add
-- ComposeT :: MonadTransformer -> MonadTransformer -> MonadTransformer
type ComposeT :: (k3 -> k2 -> Type) -> (k1 -> k3) -> (k1 -> k2 -> Type)
newtype ComposeT trans1 trans2 m a = ComposeT (trans1 (trans2 m) a)
deriving newtype (Functor, Applicative, Monad)
instance (MonadTrans trans1, MonadTrans trans2) => MonadTrans (ComposeT trans1 trans2) where
lift :: Monad m => m ~> ComposeT trans1 trans2 m
lift = ComposeT . lift . liftwhere MonadTransformer = (Type -> Type) -> (Type -> Type).
Now we can derive MonadTrans for transformers like
type OkT :: MonadTransformer
newtype OkT m a = OkT (Int -> Int -> m a)
deriving (Functor, Applicative, Monad)
via ReaderT Int (ReaderT Int m)
deriving MonadTrans
via ComposeT (ReaderT Int) (ReaderT Int)- Dominant language
- Haskell
- Stars
- 5
- Forks
- 10
- PR merge metrics
- No merged PRs in 30d
Getting set up
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
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