[Haskell-beginners] code critique

[Arlen Cuss]

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> Maybe I misunderstood you, but you aren't quite right. In third 
> solution, *list comprehension* will produce all the pairs. But
> you're applying 'null' to it, and since everything in Haskell is lazy
> by default, only *one* pair will be produced.

Good point! Lovely way to show this is with 'trace':

> :m +Debug.Trace null [x | x <- [trace "a" 5, trace "b" 10, trace "c"
> 15]]
False
> null [x | x <- [trace "a" 5, trace "b" 10, trace "c" 15], x > 0]
a
False
> null [x | x <- [trace "a" 5, trace "b" 10, trace "c" 15], x > 5]
a
b
False
> null [x | x <- [trace "a" 5, trace "b" 10, trace "c" 15], x > 10]
a
b
c
False
> 

(Of course, this is less beautifully illustrated with 'length', because
it doesn't actually evaluate the values ...:

> length [trace "a" 5, trace "b" 10, trace "c" 15] == 0
False
> 
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