Breaking down the answer to this question: https://www.reddit.com/r/haskell/comments/192d97p/just_a_simple_task_question/

haskell-eval-step-by-step.hs

concat(zipWith replicate [1,2,1,0] "cold")

-- Desugar string literal to [Char]
concat(zipWith replicate [1,2,1,0] ['c', 'o', 'l', 'd'])

-- https://hackage.haskell.org/package/base-4.19.0.0/docs/src/GHC.List.html#zipWith
-- Rewritten to not use the "go" helper, zipWith is:
-- zipWith f [] _ = []
-- zipWith f _ [] = []
-- zipWith f (x:xs) (y:ys) = f x y : zipWith f xs ys

-- Substitute zipWith:
concat(replicate 1 'c' : zipWith replicate [2,1,0] ['o', 'l', 'd'])
-- Substitute replicate:
concat(['c'] : zipWith replicate [2,1,0] ['o', 'l', 'd'])

-- Keep going..

-- Substitute zipWith:
concat(['c'] : replicate 2 'o' : zipWith replicate [1,0], ['l', 'd'])
-- Substitute replicate:
concat(['c'] : ['o', 'o'] : zipWith replicate [1,0], ['l', 'd'])

-- Substitute zipWith:
concat(['c'] : ['o', 'o'] : replicate 1 'l' : zipWith replicate [0], ['d'])
-- Substitute replicate:
concat(['c'] : ['o', 'o'] : ['l'] : zipWith replicate [0], ['d'])

-- Substitute zipWith:
concat(['c'] : ['o', 'o'] : ['l'] : replicate 0 'd' : zipWith replicate [] [])
-- Substitute replicate:
concat(['c'] : ['o', 'o'] : ['l'] : [] : zipWith replicate [] [])

-- Substitute zipWith (we've hit a base case):
concat(['c'] : ['o', 'o'] : ['l'] : [] : [])

-- Substitute concat:
['c', 'o', 'o', 'l']

-- Re-sugar into string literal:
"cool"