Optional elements and properties in Halogen

Since Halogen’s HTML, properties and classes are all arrays, dealing with optional elments might not be immediatly obvious to the less experienced Purescript coder (like me!).

For example, in most templating languages it’s possibile to write something like this (here I’m using handlebars):

Hello #{user_name}!

Obviously with Halogen we want to avoid something hideous like:

  case maybeUsername of
    Nothing -> []
    Just n -> [ HH.text $ "Hello " <> n ]

This example might look ok here, but it quickly leads to unreadable code in more real life scenarios.

Classes and properties

Monads to the rescue!

To optionally render classes and properties, we can use these operators:

join :: forall a m. Bind m => m (m a) -> m a
guard :: forall m. MonadZero m => Boolean -> m Unit
($>) :: forall f a b. Functor f => f a -> b -> f b

In our case, m is actually Array. So we can rewrite them as:

join :: forall a. Array (Array a) -> Array a
guard :: Boolean -> Array Unit
($>) :: forall a b. Array a -> b -> Array b

We are going to use guard and ($>) to return either an empty array in case the guarding fails, or an array containing just the single element when the guard succeeds.

This is because guard returns empty when the condition fails, and m Unit when it succeds. In our example, this is actually [] or [unit].

([] :: Array Unit) $> "hello" == ([] :: Array String)
[unit] $> "hello" == ["hello"]

From the example we can see that the penguin ($>) replaces the element in the array if present.

The last step is join which takes an Array (Array a) and returns Array a.

We can put all this together like this:

[ HP.classes $ join [ pure $ ClassName "hello"
                    , guard true $> ClassName "world"
                    , guard false $> ClassName "nope"

Our div will have “hello” and “world” as classes. pure in our example is [], so it’s:

join [ [ClassName "hello"], [ClassName "world"], []] 
-- which becomes
[ ClassName "hello", ClassName "world" ]

A better solution

Ok, it’s not better, but I like it more.

The problem with the previous approach is that when an optional elment has to be added, one needs to wrap all the other elements in pure and add a join.

We can abuse html to come up with a simpler solution:

dummyAttr :: forall r i. HP.IProp r i
dummyAttr = HP.attr (HH.AttrName "data-dummy") ""

whenP :: forall r i. Boolean -> HP.IProp r i -> HP.IProp r i
whenP b p = if b then p else dummyAttr

maybeP :: forall a r i. Maybe a -> (a -> HP.IProp r i) -> HP.IProp r i
maybeP m p = maybe dummyAttr p m
whenC :: Boolean -> ClassName -> ClassName
whenC b c = if b then c else ClassName ""

We add either a fake attribute or an empty class when our conditions are not met.

For example:

  [ HP.classes [ ClassName "hello"
               , whenC true $ ClassName "world"
               , whenC false $ ClassName "nope"
  , maybeP mTitle $ \t -> HP.title t             

HTML elements

A good solution most of the time is just to extract the optional HTML to a separate function:

render state =
    [ HH.text "Hello"
    , adminSection state.isAdmin
    , HH.text "World"]
adminSection false = HH.text ""
adminSection true = HH.div ...

We can write a couple of helper methods like the one we had for classes and properties:

whenH :: forall p i. Boolean -> (Unit -> HH.HTML p i) -> HH.HTML p i
whenH b k = if b then k unit else HH.text ""

maybeH :: forall a p i. Maybe a -> (a -> HH.HTML p i) -> HH.HTML p i
maybeH m k = maybe (HH.text "") k m

For example:

render state =
    [ HH.h1_ [ HH.text "Big title" ]
    , whenH state.isAdmin $ \_ -> HH.h2_ [ HH.text "Hello admin!" ]
    , maybeH state.todo $ \t -> HH.p [ HH.text $ "Remember: " <> todo ]

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