scala monad?? or function composition - scala

Question

i have this very small requirement for learning purposes.
suppose we have the following string
"1.1 This is a test 34"
where
"1.1" is the chapter
"This is a test" is the title of the chapter
"34" is the page number
The overall result should give me some indication whether or not "parsed" line is ok.
Right now its only working for "well formed" lines (this is on purpose).
So far i have 2 ways of solving this...
1) Monad approach (though im not entirely sure this is done right, therefore my question)
trait Mine[A] {
def get(): A
def map(f: A => A): Mine[A]
def flatMap(f: A => Mine[A]): Mine[A]
}
case class Attempt1[A, B](a: (A, B)) extends Mine[(A, B)] {
def get(): (A, B) = a
def map(f: ((A, B)) => (A, B)): Mine[(A, B)] = {
Attempt1(f(a._1, a._2))
}
def flatMap(f: ((A, B)) => Mine[(A, B)]): Mine[(A, B)] = {
f(a._1, a._2)
}
}
and i also have following functions for getting text out of my "string" line
def getChapter2(t: (Result, String)): Mine[(Result, String)] = {
val result = t._1
val state = t._2
result.chapter = state.substring(0, 3)
var newState = state.substring(3)
Attempt1((result, newState))
}
def getTitle2(t: (Result, String)): Mine[(Result, String)] = {
val result = t._1
val state = t._2
result.title = state.substring(0, state.length() - 2)
var newState = state.substring(state.length() - 2)
Attempt1((result, newState))
}
def getPage2(t: (Result, String)): Mine[(Result, String)] = {
val result = t._1
val state = t._2
result.page = state
Attempt1((result, ""))
}
I can think of trying to use a higher order function for code that gets values "out" of Tuple2 and creation of Attempt1 stuff, but right now i want to keep things simple, the important thing for me is the monad stuff.
And finally this is the main logic.
var line = "1.1 Some awesome book 12"
val result = new Result("", "", "")
val at1 = Attempt1((result, line))
val r = for (
o1 <- at1;
o2 <- getChapter2(o1);
o3 <- getTitle2(o2);
o4 <- getPage2(o3)
) yield (o4)
val res = r.get._1
println("chapter " + res.chapter) //1.1
println("title " + res.title) // Some awesome book
println("page " + res.page) // 12
2) Compositional approach
def getChapter(t: (Result, String)): (Result, String) = {
val result = t._1
val state = t._2
result.chapter = state.substring(0, 3)
var newState = state.substring(3)
(result, newState)
}
def getTitle(t: (Result, String)): (Result, String) = {
val result = t._1
val state = t._2
result.title = state.substring(0, state.length() - 2)
var newState = state.substring(state.length() - 2)
(result, newState)
}
def getPage(t: (Result, String)): (Result, String) = {
val result = t._1
val state = t._2
result.page = state
(result, "")
}
as u can see functions are the same except on the return type (not "wrapped" by a Mine type), and i also have this method
def process(s: String, f: ((Result, String)) => (Result, String)): Result = {
val res = new Result("", "", "")
val t = f(res, s)
res
}
and my main logic is as follows
var line = "1.1 Some awesome book 12"
var fx = getChapter _ andThen getTitle _ andThen getPage
var resx = process(line, fx)
printf("title: %s%nchapter: %s%npage: %s%n", resx.title, resx.chapter, resx.page)
returnes values are the same as the "Monad approach".
So finally the questions would be:
Is "Monad approach" really a Monad??
I find compositional approach logic easier, and for this particular case Monad approach might seem overkill but remember this is for learning purposes.
I find that in both approaches logic flow is easy to reason.
If needed in both cases is easy to add or even remove a step in order to parse a string line.
I know this code is so similar and has room for improvement but right now im keeping it simple and maybe in the future i will factor commong things out.
Suggestions are welcome.

Answer 1

First off there is no need for the vars in your code. Second, since you are using substring function of a string, all you need is one partial function which takes the offsets of the sub-strings. This would be a good place to start when refactoring and would allow varied functionality for how you split the lines should the format change.
This would look like
def splitline(method:String)(symbol:String)(s:String) = method match {
case "substring" => val symb = Integer.parseInt(symbol) ;(s.substring(0,symb),s.substring(symb))
}
val getTitle = splitline("substring")("3") _
In terms of composition or monadic code, this falls on preference and the cognitive
load you wish to place on yourself.