--- title: Hakyll 3 to Hakyll 4 migration guide author: Jasper Van der Jeugt --- Introduction ------------ This tutorial gives a quick overview on how you can port your blog/website from Hakyll 3.X to Hakyll 4. A lot of changes have happened, so it might be useful to read through the [tutorial series](/tutorials.html) before porting your website. Arrow becomes Monad ------------------- In Hakyll 3.X, `Compiler` was an instance of `Arrow`. Since Hakyll 4, `Compiler` is a `Monad`. This means that previous chains such as: ```haskell compile $ someCompiler >>> someOtherCompiler >>> anotherCompiler ``` Now take the general form of: ```haskell compile $ someCompiler >>= someOtherCompiler >>= anotherCompiler ``` Page goes away -------------- The `Page` type in Hakyll 3.X has been removed and replaced by an `Item` type. `pageCompiler` no longer exists -- where you previously used this, you probably want to use `pandocCompiler` instead. `Page`s were manipulated using `setField`/`getField` functions in Hakyll 3.X. In Hakyll 4, all metadata is completely immutable, so these functions have been removed. In order to format and add fields, use a `Context` -- see the next section. Template changes ---------------- The template format has become slightly more flexible, whereas in Hakyll 3.X only keys such as this were allowed: ```html

$title$

``` we now allow arbitrary strings. This will be really useful in the future. ```html

$uppercase title$

``` Some template functions have been renamed: - `applyTemplateCompiler` becomes: `loadAndApplyTemplate` - `applySelf` becomes: `applyAsTemplate` Instead of setting fields in a `Page` before applying a template, we now use a `Context`. More information on context can be found in [this tutorial](/tutorials/04-compilers.html). For migration, you basically want to map every `setField` to a field in a `Context`. Metacompilers go away --------------------- For tags, the [Hakyll.Web.Tags] module still provides a solution. In other cases, the `preprocess` function should be able to compensate for this. [Hakyll.Web.Tags]: /reference/Hakyll-Web-Tags.html