| On pattern guards, Simon PJ writes:
| > f (x:xs) | x<0 = e1
| > | x>0 = e2
| > | otherwise = e3
|
| then
| > g a | (x:xs) <- h a, x<0 = e1
| > | (x:xs) <- h a, x>0 = e1
| > | otherwise = e3
|
| Am i right in thinking that f [] is bottom, whilst g [] is e3?
Not quite. Certainly f [] is bottom. But g matches (x:xs) against (h a),
not against a. So a might not be of type list. What is certainly true
is that if (h a) == [], then g a is e3.
| Later, another definition of g (intended to be equivalent?) is given in
| which g [] appears to be bottom:
| > g a | (x:xs) <- h a
| > | x<0 = e1
| > | x>0 = e2
| > | otherwise = e3
|
| To match the semantics of the earlier definition of g, shouldn't this
| read as follows?
| g a | (x:xs) <- h a
| | x<0 = e1
| | x>0 = e2
| | otherwise = e3
You're dead right. Nested guards are a bit tricky, eh?
Simon
