404 on Haddock page in `base`?

winter · February 26, 2026, 3:02am

Hackage’s Haddock page for base’s Control.Applicative.optional* tries to link to Control.Monad.Except, but the resulting URL isn’t valid.

The link looks to be generated based on the comment including the item name in quotes, but Haddock isn’t properly linking to Except in Control.Monad, instead trying to link to the Control.Monad.Except. What’s the proper way to link to this, and how easy is it to submit an MR to base to fix this? Is it worth fixing? Thanks!

*I imagine this isn’t how you spell the fully-qualified name for an item — corrections appreciated!
** I tried to add this as a proper footnote, but Discourse thought it was a link (dunno why) and prevented me from posting it due to my TL.

taylorfausak · February 26, 2026, 4:22am

It is trying to link to the module Control.Monad.Except, which is defined in the mtl package. The link is syntactically valid, but trying to link to the wrong place for some reason. The link target should be https://hackage-content.haskell.org/package/mtl-2.3.2/docs/Control-Monad-Except.html.

github.com/ghc/ghc

libraries/base/src/Control/Applicative.hs

fa88d09ae


      
                   => Data (WrappedArrow a b c)
          
          -- extra functions
          
          -- | One or none.
          --
          -- It is useful for modelling any computation that is allowed to fail.
          --
          -- ==== __Examples__
          --
          -- Using the 'Alternative' instance of "Control.Monad.Except", the following functions:
          --
          -- >>> import Control.Monad.Except
          --
          -- >>> canFail = throwError "it failed" :: Except String Int
          -- >>> final = return 42                :: Except String Int
          --
          -- Can be combined by allowing the first function to fail:
          --
          -- >>> runExcept $ canFail *> final
          -- Left "it failed"

github.com/haskell/mtl

Control/Monad/Except.hs

fb3a0afd8


      
          The Error monad (also called the Exception monad).
          
          @since 2.2.1
          -}
          
          {-
            Rendered by Michael Weber <mailto:michael.weber@post.rwth-aachen.de>,
            inspired by the Haskell Monad Template Library from
              Andy Gill (<http://web.cecs.pdx.edu/~andy/>)
          -}
          module Control.Monad.Except
            (
              -- * Warning
              -- $warning
              -- * Monads with error handling
              Error.MonadError(..),
              Error.liftEither,
              Error.tryError,
              Error.withError,
              Error.handleError,
              Error.onError,

Leary · February 26, 2026, 10:38am

As noted in the haddock documentation for module links:

[…] whether the module is in scope isn’t checked and will always be turned into a link.

Since base does not depend on mtl, the target module presumably isn’t in scope. I suspect there’s no better option than to spell out the link manually:

[Control.Monad.Except](https://hackage.haskell.org/package/mtl/docs/Control-Monad-Except.html)

jeffstyr · February 26, 2026, 6:56pm

I guess how it works must be that if it’s not in scope, it assumes the module is in the same package, rather than giving up. It would be nice if there were a way to indicate the package in the Haddock, other than using the full URL, but it doesn’t look like there is.