String sequence in Haskell
The primary issue I see is that your code is focused on the wrong sort of state. What you have written would be used something like&mdash;

iterate (getNextStr (Sequence "abc" 'a' 'c')) "a"


But this is very slow, you can see from the repeated usage of last, (++), and reverse that the actual state that matters (which character is next in the sequence) is getting parsed out of the previous state anew with every iteration. You are relying on repeating many \$O(n)\$ operations, which ends up causing your code to have a terrible constant factor performance hit.

The key insight or trick is to take advantage of lazy evaluation to produce an infinite stream of values which can hide all of the messy state machinery you need away in some thunks.

stringSequence :: [Char] -&gt; [String]


We start with a clean slate and a new type signature. In this case we have a function that takes a sequence of Char values, and returns a list (which we know will be infinite) of String values made by applying the sequence generating procedure to the Chars. The magic obviously happens in the sequence generating procedure, so let's concentrate on that.

stringSequence chars = map (:"") chars


Starting is easy, we simply make a String out of each Char in our given sequence. Next step is to generate the two character Strings.

stringSequence chars =
  let
    strings = map (:"") chars
    front   = strings
    next    = concatMap (\f -&gt; map (f ++) strings) front
  in
    front ++ next


This is the biggest jump I'll make, so make sure you understand it. The two character strings are made by taking each string we've already generated and appending a Char from the sequence to it (for convenience, strings makes it easy to combine previous values generated by the sequence generating function and the next element of the given Chars).

stringSequence chars =
  let
    strings = map (:"") chars
    front   = strings
    next    = concatMap (\f -&gt; map (f ++) strings) front
    after   = concatMap (\s -&gt; map (s ++) strings) next
  in
    front ++ next ++ after


Now we add the three character strings, and notice a pattern. Strings of length \$n+1\$ are made by appending the sequence characters to the strings of length \$n\$. Leveraging this insight and a trick called Tying the Knot, we can write the final version of stringSequence that produces an infinite lazy list of sequenced values.

stringSequence chars =
  let
    strings = map (:"") chars
    sequence = strings ++ concatMap (\s -&gt; map (s ++) strings) sequence
  in
    sequence


Compare this version to your original. The Sequence type you had is now replaced with a simple list. Instead of getNextChar we use the whole sequence in one go so we don't have to perform additional bookkeeping to maintain correct ordering. Rather than parsing the previous result value with getNextStr, we maintain our state hidden away from the user by providing them an infinite list, instead of out in the open where the user has to keep track of it and we have to keep parsing it.