Swap even/odd characters
The solution should look like (pseudocode, I know little F# too ;) ):

String
  |&gt; ChunksOf 2
  |&gt; Map Reverse
  |&gt; Flatten


An example explains clearly

"abcd" -&gt; ["ab", "cd"] -&gt; ["ba", "dc"] -&gt; "badc"


The key of effective Functional Programming is making good use of the built-in functions and carefully combining them.