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Special Haskell Syntax used by IHP

IHP uses many “exotic” haskell features. Here’s a short explanation of the most common. In case you think something is missing, let us know on Slack.

The hash symbols #

IHP uses hash symbols # all over the place, like in this code:

set #companyId (get #companyId company)

The hashes are provided by the OverloadedLabels language extension.

In IHP you can think of these hash-strings as immutable type-level strings. One of the most common uses for labels is to represent field names. In the above example #companyId refers to the companyId field on the Company data structure.

Technical details:

Writing #companyId is equivalent to writing fromLabel @"companyId". The fromLabel function is provided by a type class and can be implemented by e.g. libraries and frameworks. In IHP this instance is usually this:

instance IsLabel name (Proxy name') where
    fromLabel = Proxy @name'

So #companyId can be written as fromLabel @"companyId" which IHP turns into Proxy @"companyId". The Proxy value is now a normal haskell value and is passed to functions such as get or set.

The at symbol @

Another symbol used often in IHP is the @ symbol, like this:

action UsersAction = do
    users <- query @User |> fetch
    render IndexView { .. }

This is called a Type application.

There’s a great blog post by IHP contributor Zac Wood explaining Type applications in IHP. It’s best to just read the article :)

If you have open questions about lists like fill @["title", "body"], after reading the linked blog post:

These are type-level lists. You can write @"type level strings" and type level integers like @1337, so you can also write type level lists like @["title", "body"].

Be aware: When you write a type level list like @["title"], so with only a single element, you need to prepend a ' like this @'["title"]. Otherwise you’ll get an error. We opened a ticket on the haskell compiler for this already.

The { .. }

Inside actions the views are ususally rendered like this:

action UsersAction = do
    users <- query @User |> fetch
    render IndexView { .. }

We asume that the view is defined like this:

data IndexView = IndexView { users :: [User] }

Then the expression IndexView { .. } is a shortcut for IndexView { users = users }.

When the haskell compiler expands this, it will internally be like this:

action UsersAction = do
    users <- query @User |> fetch
    render IndexView { users = users }

You can learn more here about these so-called Record wildcards.

The bang operator !

Inside your Types.hs you see lots of !s, like this:

ShowPostAction { postId :: !(Id Post) }

The ! marks the postId field as strict. Strict means that the value of postId is not stored as a lazy value. So instead of only computing the postId field when needed, it will already be fully computed when the ShowPostAction value is constructed. Basically we tell haskell that we’re sure that this field is always needed. This makes the action data structures more memory efficient.

You can learn more about strictness and lazyness in haskell in this blog post.

The \case

Inside controllers you typically find code like this:

user
    |> fill @["firstname", "lastname"]
    |> validateField #firstname nonEmpty
    |> ifValid \case
        Left user -> render EditView { .. }
        Right user -> do
            user <- user |> updateRecord
            redirectTo EditUserAction { .. }

The \case is called a Lambda case in haskell. It’s a shortcut for writing this:

user
    |> fill @["firstname", "lastname"]
    |> validateField #firstname nonEmpty
    |> ifValid (\user -> case user of
            Left user -> render EditView { .. }
            Right user -> do
                user <- user |> updateRecord
                redirectTo EditUserAction { .. }
        )

In general the \case ... can be expanded to \value -> case value of ....

Learn more about lambda cases here.

The Pipe operator |>

In IHP code bases you find lot’s of usage of the |> operator. We usually call it the pipe operator.

The operator allows you to write code like this:

user
    |> fill @["firstname", "lastname"]
    |> validateField #firstname nonEmpty

Instead of the normal function style:

validateField #firstname nonEmpty (
        fill @["firstname", "lastname"] user
    )

In general:

function arg1 arg2 object 
=
object |> function arg1 arg2

The operator is itself is defined as a haskell function inside IHP.HaskellSupport:

infixl 8 |> -- This tells haskell to theat the |> as a infix operator
a |> f = f a -- This is the actuall implementation

Tell GHC(Haskell Compiler) To Infer Constraints And Implicit Parameters

Let’s say you are working with a controller ApplicationsAction and most actions have similar access control:

    action NewApplicationAction { jobPositionId } = do
        jobPosition <- fetch jobPositionId

        -- Access Control
        jobPositions <- currentCompanyJobPositions
        accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))

        ...

    action UpdateApplicationAction { applicationId } = do
        application <- fetch applicationId

        -- Access Control
        jobPositions <- currentCompanyJobPositions
        accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))

        ...

We could start by refactoring the access control logic into a function:

accessDeniedUnlessJobPositionAllowed jobPosition = do
    jobPositions <- currentCompanyJobPositions
    accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))

And then add a type declaration:

accessDeniedUnlessJobPositionAllowed :: JobPosition -> IO ()
accessDeniedUnlessJobPositionAllowed jobPosition = do
    jobPositions <- currentCompanyJobPositions
    accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))

However, GHC will give us an error message stating:

...

Application/Helper/Controller.hs:51:21: error:
    * Unbound implicit parameter (?context::ControllerContext)
        arising from a use of `currentCompanyJobPositions'
    * In a stmt of a 'do' block:
        jobPositions <- currentCompanyJobPositions
      In the expression:
        do jobPositions <- currentCompanyJobPositions
           accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))
      In an equation for `accessDeniedUnlessJobPositionAllowed':
          accessDeniedUnlessJobPositionAllowed jobPosition
            = do jobPositions <- currentCompanyJobPositions
                 accessDeniedUnless (get #id jobPosition `elem` (ids jobPositions))
   |
51 |     jobPositions <- currentCompanyJobPositions
   |

We could explicitly add the ?context::ControllerContext implicit:

accessDeniedUnlessJobPositionAllowed :: (?context::ControllerContext) => JobPosition -> IO ()

Writing out implicit parameters, and other type constrains could become messy and/or irritating, so we could tell GHC to infer it:

accessDeniedUnlessJobPositionAllowed :: _ => JobPosition -> IO ()