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Learn You a Haskell shows the intersperse function:
ghci> intersperse '.' "MONKEY"
"M.O.N.K.E.Y"
ghci> intersperse 0 [1,2,3,4,5,6]
[1,0,2,0,3,0,4,0,5,0,6]
Note that the interspersed element only appears inside of the elements.
list elem ... item ... list elem ..... item .. list ele...