Editing Handling errors in Haskell (section)

== Error (pure code) ==

Some pure functions contain calls to error, causing the evaluation to stop and crash:

<haskell>
head :: [a] -> a
head (x:_) = x
head []    = error "empty list"
</haskell>

Now suppose someone writes <hask>head ages</hask> and unexpectedly, <hask>ages</hask> is an empty list. If you are trying to take the head of an empty list your program logic is simply broken. 

A solution here is to avoid the <hask>head</hask> function and use listToMaybe from [http://hackage.haskell.org/package/base/docs/Data-Maybe.html Data.Maybe].

<haskell>
case listToMaybe ages of
  Nothing -> defaultAge
  Just first -> first
</haskell>

Alternatively, these errors can be caught from IO monad by using <hask>evaluate</hask> and <hask>try</hask> from [http://hackage.haskell.org/package/base/docs/Control-Exception.html Control.Exception]. Ideally you should avoid partial functions like <hask>head</hask>, but sometimes this is not an option (e.g. when using an external library)