run length encoding with a fold

rle.fsx

//remove consecutive duplicates
let uniq items = 
    let uniq accum item = 
        match accum with 
        | a::_ when a = item -> accum 
        | _ -> item::accum
    items |> List.fold uniq [] |> List.rev
    

let uniq_tests = 
  [
    uniq [] |> List.isEmpty
    uniq ['a'] = ['a']
    uniq ['a';'b'] = ['a';'b']
    uniq ['a';'a'] = ['a']
  ]
  
//run length encoding
let rle items =
    let rle accum item = 
        match accum with
        | (count,a)::rest when a=item -> (count+1,item)::rest
        | _ -> (1,item)::accum 
    items |> List.fold rle [] |> List.rev

let rle_tests = 
   [
    rle [] = []
    rle ['a'] = [(1,'a')]
    rle ['a';'b'] = [(1,'a');(1,'b')]
    rle ['a';'a'] = [(2,'a')]
    rle ['a';'b';'b';'c'] = [(1, 'a'); (2, 'b'); (1, 'c')]
  ]
// if we sort, we have histogram

["hello";"world";"hello"] |> List.sort |> rle