CODEWITHSUNDEEP
purescriptoption-type
bensnowball
bensnowball

Reputation: 59

Nested maybe and nullable

Having trouble untangling maybe and nullable values in the following:

container <-
  unsafePartial
  (fromJust <<< toMaybe
  <$> DOM.querySelector (DOM.QuerySelector "body")
  (DOM.htmlDocumentToParentNode document))

Which gives me the following error:

Could not match type

Maybe

with type

Nullable

while trying to match type Maybe Element
with type Nullable t1
while checking that expression (querySelector (QuerySelector "body"))
(htmlDocumentToParentNode document)
has type t0 (Nullable t1)
in value declaration main

where t0 is an unknown type
t1 is an unknown type

I've tried but I can't find my way through the various types being used here (code is originally from here)

Upvotes: 2

Views: 158

Answers (1)

Phil Freeman
Phil Freeman

Reputation: 4169

I would recommend breaking this up a bit.

Start with

do body <- DOM.querySelector (DOM.QuerySelector "body")
             (DOM.htmlDocumentToParentNode document))
   ?whatNext

Here, ?whatNext is a typed hole. The compiler will tell you the inferred type of the hole, which should help you to figure out what to replace it with.

Also note that you can simplify things a little by applying unsafePartial to fromJust directly, to give a function of type Maybe a -> a.

Upvotes: 1

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