A recursive function is tail recursive if the final result of the recursive call is the final result of the function itself. If the result of the recursive call must be further processed (say, by adding 1 to it, or consing another element onto the beginning of it), it is not tail recursive.
With that said, tail recursion is not that useful of a concept in a
lazy language like Haskell. The important concept to know in Haskell
is guarded recursion, where any recursive calls occur within a data
constructor (such as
foldr, where the recursive call to foldr occurs
as an argument to
(:)). This allows the result of the function to be
consumed lazily, since it can be evaluated up to the data constructor
and the recursive call delayed until needed.
Note that foldl is always tail recursive.
- Brent Yorgey in Haskell-Cafe on Definition of "tail recursive" wrt Folds