[CH4] A more concise implementation of fibTailRec.

[sandeep-datta]

Hi,

What do you think of the following implementation of fibTailRec from CH4?

fibTailRec :: Int -> Int
fibTailRec n = fibtr n 0 1 where
  fibtr :: Int -> Int -> Int -> Int
  fibtr n a b =
    if n == 0 then a
    else fibtr (n-1) b (a+b)

[milesfrain]

I like it!

I think we should still keep the existing verbose and beginner-friendly reference solution, but you're welcome to make a PR to add your more optimized solution too.

Just add it after the reference solution here:

purescript-book/exercises/chapter4/test/no-peeking/Solutions.purs

Lines 76 to 86 in 427f698

	fibTailRec :: Int -> Int
	fibTailRec 0 = 0
	fibTailRec 1 = 1
	fibTailRec n = fib' n 2 0 1
	where
	fib' :: Int -> Int -> Int -> Int -> Int
	fib' limit count n1 n2 =
	if limit == count then
	n1 + n2
	else
	fib' limit (count + 1) n2 (n1 + n2)

Could call it fibTailRec2. Might be good to also include a comment describing that this is another way to do it by "counting down".

And be sure to test it:

purescript-book/exercises/chapter4/test/Main.purs

Lines 147 to 156 in 427f698

	suite "Exercise - fibTailRec" do
	test "Verify 0" do
	Assert.equal 0
	$ fibTailRec 0
	test "Verify 9" do
	Assert.equal 34
	$ fibTailRec 9
	test "Verify 44" do
	Assert.equal 701408733
	$ fibTailRec 44

You could make a helper within that suite which calls both versions, and then replace the calls to each test case with a call to the helper.

Similar to what's done for primeFactors, although that helper is just testing one solution.

purescript-book/exercises/chapter4/test/Main.purs

Lines 126 to 134 in 427f698

	suite "Exercise - primeFactors" do
	let
	primeFactorsTest :: Int -> Array Int -> _
	primeFactorsTest n xs =
	test (show n) do
	Assert.equal (sort xs)
	$ sort
	$ primeFactors n
	primeFactorsTest 1 []

Note that you'll also need to make sure that the test for your optimized reference solution is deleted when users call prepareExercises.sh, since we don't expect both versions to be available when users run their tests. You can add a -- This line should have been automatically deleted comment after the call to your function in the helper.

purescript-book/scripts/prepareExercises.sh

Lines 19 to 20 in 427f698

	# Delete lines with a note to delete them
	perl -ni -e 'print if !/This line should have been automatically deleted/' $f

Might be good to increase coverage too for inputs 1, 2, and 3.