Daryoush Mehrtash wrote:
Is the call to "go" in the following code considered as tail recursion?
data DList a = DLNode (DList a) a (DList a)
mkDList :: [a] -> DList a
mkDList [] = error "must have at least one element"
mkDList xs = let (first,last) = go last xs first
in first
where go :: DList a -> [a] -> DList a -> (DList a, DList a)
go prev [] next = (next,prev)
go prev (x:xs) next = let this = DLNode prev x rest
(rest,last) = go this xs next
in (this,last)
No. For @go _ (_:_) _@ the tail expression is @(this,last)@ and so the
tail call is to @(,)@. Consider this general transformation[1]:
go prev [] next = (next,prev)
go prev (x:xs) next =
case DLNode prev x rest of this ->
case go this xs next of (rest,last) ->
(this,last)
Let binding is ignored when determining tail-callingness, and case
evaluation only contributes in as far as allowing multiple tails.
[1] Which isn't laziness-preserving and so will break on your recursive
let binding. It's a valid transformation for non-recursive let bindings,
though, provided the appropriate strictness analysis.
--
Live well,
~wren
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
