At the last minute, I have found some Enum bugs in 1.2.beta.
As I don't know if they have been fixed in 1.2.gamma, which
I guess was circulated yesterday, here they are:
1) According to the specification (section 3.9), the
value of
[ 7, 7 .. 3 ]
is the infinite list [ 7, 7, 7, 7,...], but the
default definition of enumFromThenTo (pp. 30 and 91)
gives the value
takeWhile (<= 3) enumFromThen 7 7
which is the empty list!
2) Similarly, the value of
['z', 'y' .. 'a']
should be the alphabete in reverse order, but according to
the Enum instance of Char (p. 92), we get
takeWhile (>= 'a') (map chr [ord 'z', ord 'y' .. ord maxChar]) ==
takeWhile (>= 'a') (map chr []) ==
[],
i.e, also the empty list.
Suggested fixes:
1) The default definition of enumFromThenTo (pp. 30 and 91)
should be:
enumFromThenTo n n' m | n < n' = takeWhile (<= m) (enumFromThen n n')
| n > n' = takeWhile (>= m) (enumFromThen n n')
| n == n' = repeat n
| otherwise = error "enumFromThenTo{PreludeCore}: \
\no ordering relation"
2) The instance Enum Char (p. 92) should read:
enumFromThen c c' | c < c' = map chr [ord c, ord c' .. ord maxChar]
| c > c' = map chr [ord c, ord c' .. ord minChar]
| otherwise = repeat c
-- Mikael R.
