[Haskell-beginners] do-notation question

[Ertugrul Soeylemez]

blackcat at pro-ns.net wrote:

>  In The Yesod Book, I see the following code:
>
>  defaultLayout contents = do
>          PageContent title headTags bodyTags <- widgetToPageContent $ do
>              addCassius ...
>
>  How does this work?  Is bodyTags drawn from widgetToPageContent and
>  then used as the third argument to PageContent?

This is just a regular pattern match, just like any of these:

  x         <- someComp
  (x, y)    <- someTupleComp
  Just x    <- someMaybeComp
  [x, y, z] <- someListComp
  x : y : _ <- someListComp
  PageContent title headTags bodyTags <- somePageContentComp

or:

  someFunc (PageContent title headTags bodyTags) = ...
  someFunc (Just x) = ...

What may confuse you is that PageContent is used in prefix notation
without any parentheses, whereas the usual constructors used in pattern
matching are written in infix or special syntax notation.


Greets,
Ertugrul


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