John Meacham :
The fact that bottom is a value in Haskell is the fundamental thing that differentiates Haskell from other languages and the source of its power. The fact that f _|_ /= _|_ potentially _is_ what it means to be a lazy language. Not treating _|_ as a value would be a huge disservice to anyone learning the language. Sure, it may seem a little strange coming from the imperative world to think of it as a value, but it is by far the oddest concept in Haskell, after all, _functions_ are values in Haskell and people seem to eventually figure that out. Well... Personally I hate thinking about bottom as "value". I don't do this. I NEVER teach that. And, I am a "lazy guy", almost all my Haskell programs are strongly based on laziness.
I'll tell you what I teach, and you might throw some tomatoes... "The fundamental thing that differentiates Haskell from other languages and the source of it power" - if I might cite you - is that we don't see the difference between an object and the process which creates it. (The difference demands that we speak about the call-by-need, etc...) The bottom, as sin (2*pi), or "Text" may be seen as processes. Anyway, a lazy list IS a process /par excellence/. The _|_ entity is a process which refuses to give you a value (or does it in a deadly way). Your program manipulates processes. A process in some computational context must do something - or not. The bottom never does anything useful. All this is probably a question of language, of terminology, of preconceptions (of all that, what for God knows which reasons, Americans call "just semantics"), but I will not forget the day when I thought as you, and I had to explain to 2-nd year students what does it mean: a value which doesn't have a value... Thank you. Jerzy Karczmarczuk