Editing Handling errors in Haskell (section)

== No value using the Maybe type ==

There is simply no value there. This isn't a problem in the system. It means you don't care why there isn't a value, or you already know. <hask>Maybe a</hask>

Typical example:

<haskell>
lookup :: Eq a => a -> [(a,b)] -> Maybe b
</haskell>

That is, take some key <hask>a</hask> that can be compared for equality, and a list of pairs where the first is the same type of the key <hask>a</hask> and maybe return the <hask>b</hask> of the pair, or nothing.

So one might pattern match on this:

<haskell>
case lookup name person of
  Nothing -> "no name specified"
  Just name -> "Name: " <> name
</haskell>

Or use a deconstructing function:

<haskell>
maybe "no name specified"
      ("Name: " <>)
      (lookup name person)
</haskell>

Again, depends on the code and the person writing it whether an explicit case is used. Often monads like <hask>Maybe</hask> are composed to make a chain of possibly-nothing values:

<haskell>
lookup "height" profile >>=
parseInt >>=
flip lookup recommendedSizes
</haskell>


So lookup a height from a person's profile (might not exist), parse it as integer (might not parse), then use that as a key to lookup from a mapping list of age to clothes size: <hask>[(Int,Text)]</hask>