Trying to add metadata for urls for next and previous links but can't resolve error

[Abner Gershon]

I am seeking to create a series of "slides", ie webpages, to be used in a presentation. I thought this might be a good way to get to know Hakyll a bit.

Here is the fragment of code that I use to define the slide context, slideCtx

---
slideCtx :: Context String
slideCtx =
    field "prevSlide" $ \item -> do
        metadata <- getMetadata (itemIdentifier item)
        return $ maybe "" id $ lookupString "prevSlide" metadata
    `mappend`
    field "nextSlide" $ \item -> do
        metadata <- getMetadata (itemIdentifier item)
        return $ maybe "" id $ lookupString "nextSlide" metadata
    `mappend`
    defaultContext

---

This results in the following error message:

site.hs:79:5: error: [GHC-83865]
    • Couldn't match expected type: Compiler String
                  with actual type: Context String
    • In the second argument of ‘mappend’, namely ‘defaultContext’
      In the expression:
        do metadata <- getMetadata (itemIdentifier item)
           return $ maybe "" id $ lookupString "nextSlide" metadata
          `mappend` defaultContext
      In the second argument of ‘($)’, namely
        ‘\ item
           -> do metadata <- getMetadata (itemIdentifier item)
                 return $ maybe "" id $ lookupString "nextSlide" metadata
                `mappend` defaultContext’
   |
79 |     defaultContext
   |     ^^^^^^^^^^^^^^
(ERROR)-(Exit Code 1)-(General error)
--- full site.hs attached attached below

I get that there is a type error, specifically that defaultContext has type Context String and should be Compiler  String. Just not sure how to resolve this. I have been struggling with this for quite a while now and really have no clue how to fix this.

Gratefully,

Abner

[Alexander Batischev]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello Abner,

On Wed, Jan 01, 2025 at 06:29:35PM -0800, Abner Gershon wrote:
> In the expression:
> do metadata <- getMetadata (itemIdentifier item)
> return $ maybe "" id $ lookupString "nextSlide" metadata
> `mappend` defaultContext

Apparently the compiler thinks that ``` `mappend` defaultContext ``` is
part of the second argument to `field "nextSlide"`. I think it happens
because `($)` takes the rest of the expression as the argument.

I believe you can fix this with some parentheses:

```

slideCtx :: Context String
slideCtx =

(field "prevSlide" $ \item -> do

metadata <- getMetadata (itemIdentifier item)

return $ maybe "" id $ lookupString "prevSlide" metadata)
`mappend`
(field "nextSlide" $ \item -> do

metadata <- getMetadata (itemIdentifier item)

return $ maybe "" id $ lookupString "nextSlide" metadata)
`mappend`
defaultContext
```

`mconcat` might be more readable:

```

slideCtx :: Context String
slideCtx =

mconcat
[

field "prevSlide" $ \item -> do
metadata <- getMetadata (itemIdentifier item)
return $ maybe "" id $ lookupString "prevSlide" metadata

, field "nextSlide" $ \item -> do

metadata <- getMetadata (itemIdentifier item)
return $ maybe "" id $ lookupString "nextSlide" metadata

, defaultContext
]
```

- --
Regards,
Alexander Batischev

PGP key 356961A20C8BFD03
Fingerprint: CE6C 4307 9348 58E3 FD94 A00F 3569 61A2 0C8B FD03

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[Abner Gershon]

Thanks so much. This solution works like a charm.