Hello! Somebody posted a list of Morse "palindromes" on twitter which got me thinking about the timing of Morse. I think this page has the right take on the palindrome question: http://scruss.com/blog/2013/10/12/morse-palindromes-or-cq-christian-bok/ and I'm playing with this in Haskell.
I have a comment on your use of "concat [dit, space]" to create inter-symbol pauses. It seems to introduce a spurious space at the end of each word. Why not use "intersperse" instead to insert space, midSpace and longSpace where appropriate? So letters would be assembled from lists of dit, dah (interspersing space), words are assembled from lists of letters (interspersing medSpace) and the sentence is assembled from a list of words (interspersing long space)?