GoldPython wrote:
I know there's "length" to count the elements in a list and it works fine, but the function below stack overflows on a large list.
countLines [] = 0 countLines (_:ls) = 1 + countLines ls
I would have thought that this was tail recursive and would be flattened into iteration by the compiler.
This is not tail recursive as written. SICP section 1.2.1 does a good job of explaining how to tell the difference:
http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-11.html#%_sec_1.2.1
The analysis in Haskell is a bit different in general because reductions are applied in a different order, but in this case it's exactly the same.
Some compilers might indeed optimize your code into a tail-recursive version, but there's a catch: the optimization depends on the commutativity and associativity of (+), which doesn't hold for arbitrary types in Num. Try giving countLines an explicit type signature like ([a] -> Int) and see if that helps.
-- Ben
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