title: Haskell pattern match "diverge" and ⊥
body: <p>I'm trying to understand the <a href="http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17" rel="noreferrer">Haskell 2010 Report section 3.17.2</a> "Informal Semantics of Pattern Matching".  Most of it, relating to a pattern match succeeding or failing seems straightforward, however I'm having difficulty understanding the case which is described as the pattern match "diverging".</p>
  
  <p>I'm semi-persuaded it means that the match algorithm does not "converge" to an answer (hence the match function never returns).  But if doesn't return, then, how can it return a value, as suggested by the parenthetical "i.e. return <code>⊥</code>"?  And what does it mean to "return <code>⊥</code>" anyway? How one handle that outcome? </p>
  
  <p>Item 5 has the particularly confusing (to me) point "If the value is <code>⊥</code>, the match diverges".  Is this just saying that a value of <code>⊥</code> produces a match result of <code>⊥</code>? (Setting aside that I don't know what that outcome means!)</p>
  
  <p>Any illumination, possibly with an example, would be appreciated!</p>
  
  <hr>
  
  <p>Addendum after a couple of lengthy answers:
  Thanks Tikhon and all for your efforts. </p>
  
  <p>It seems my confusion comes from there being two different realms of explanation: The realm of Haskell features and behaviors, and the realm of mathematics/semantics, and in Haskell literature these two are intermingled in an attempt to explain the former in terms of the latter, without sufficient signposts (to me) as to which elements belong to which.  </p>
  
  <p>Evidently "bottom" <code>⊥</code> is in the semantics domain, and does not exist as a value within Haskell (ie: you can't type it in, you never get a result that prints out as " <code>⊥</code>"). </p>
  
  <p>So, where the explanation says a function "returns  <code>⊥</code>", this refers to a function that does any of a number of inconvenient things, like not terminate, throw an exception, or return "undefined". Is that right?</p>
  
  <p>Further, those who commented that  <code>⊥</code> actually <em>is</em> a value that can be passed around, are really thinking of bindings to ordinary functions that haven't yet actually been called upon to evaluate ("unexploded bombs" so to speak) and might never, due to laziness, right?</p>