ANN: Hakyll 4.8

[Jasper Van der Jeugt]

Hi all,

I've just released Hakyll-4.8. The main change is that we now use the
yaml package to parse page metadata. This was a frequently requested
feature and it will allow for having more structure in your metadata.

The metadata type changed from:

type Metadata = Map String String

To:

type Metadata = Aeson.Object

Most configurations do not rely on the internals of this object, so
this should not pose a big problem.

What can happen is that your metadata contains invalid YAML, e.g.:

---
title: My first blogpost: Hello world
---

The ':' character is invalid, and you would have to change this to:

---
title: 'My first blogpost: Hello world'
---

Hakyll will generate a nice error message if you have this problem
anywhere in your posts, so this should be easy to fix.

Let me know if you run into any trouble.

Peace,
Jasper

[Sergey Bushnyak]

Just switched to the 4.8 and needed to apply changes. I'm using Metadata heavily to manipulate different parts based of page based on meta, some parts of it considered optional. 

For those who will also need changes, here is an example code. 

```

titleCtx :: Context a

titleCtx =

  field "ptitle" $ \item ->

    do

      metadata <- getMetadata $ itemIdentifier item

      let title =

            case Map.lookup "title" metadata of

              Just value -> "Sigrlami - " ++ value

              Nothing    -> "Sigrlami" 

      return title

```

will change to

```

import              Data.Aeson.Types

import qualified Data.HashMap.Strict as HM

titleCtx :: Context a

titleCtx =

  field "ptitle" $ \item ->

    do

      metadata <- getMetadata $ itemIdentifier item

      let title =

              case HM.lookup "title" metadata of

                Nothing    -> ""

                Just value ->   -- this is not actual value, case it again

                  case value of

                    String x -> "Sigrlami - " ++ T.unpack x 

                    _          -> "Sigrlami"

      return title

```

and you'll need to add `aeson` and `unordered-containers` to your cabal dependencies.

Hope this will help someone.

[k...@eunchan.net]

Hi,

What I did was to use 'lookupString' function inside Hakyll.Core.Metadata 

Here's excerpt from the code:

-- | Filter Public for match funciton
-- Usage : matchMetadata "pattern/**" postIsPublic
metadataFieldIs :: String -> String -> Metadata -> Bool
metadataFieldIs key value metadata =
    case lookupString key metadata of
        Just v  -> value == v
        Nothing -> False

Regards,

Eunchan