Chapter 4 Accumulators example doesn't trigger TCO. about purescript-book HOT 3 CLOSED

Thanks a bunch for reporting this and providing an explanation of what's happening.
PR to fix in #97

from purescript-book.

So the example has been updated for better style and to remove warnings. However, this issue still occurs and the parenthesis still fix it. While this is a simple fix, I feel like it would be nice to provide an explanation. Sadly, I don't myself understand it.

from purescript-book.

The example isn't correct; it needs the parentheses because of precedence rules (the function is being applied to acc and then we add one to the result).

This code:

length' (fromMaybe [] $ tail arr) acc + 1

is parsed like this:

(length' (fromMaybe [] $ tail arr) acc) + 1

instead of the way it's intended from my reading of the example, which is:

length' (fromMaybe [] $ tail arr) (acc + 1)

Just to check this, I threw it into try.purescript.org and looked at the output, and indeed the version without parenthesis emits this JS:

return length$prime(Data_Maybe.fromMaybe([  ])(Data_Array.tail(arr$prime)))(acc) + 1 | 0;

which is adding 1 to the result of length' after it is called on acc, which isn't going to fire the TCO (or increment the accumulator).

So I'd suggest just fixing the example without further explanation -- it's not that this is a tricky case, it's just that the example is incorrect.

from purescript-book.