I'm building comfort going through some Haskell toy problems and I've written the following speck of code
multipOf :: [a] -> (Int, a)
multipOf x = (length x, head x)
gmcompress x = (map multipOf).group $ x
which successfully preforms the following operation
gmcompress [1,1,1,1,2,2,2,3] = [(4,1),(3,2),(1,3)]
Now I want this function to instead of telling me that an element of the set had multiplicity 1, to just leave it alone. So to give the result [(4,1),(3,2),3] instead. It be great if there were a way to say (either during or after turning the list into one of pairs) for all elements of multiplicity 1, leave as just an element; else, pair. My initial, naive, thought was to do the following.
multipOf :: [a] -> (Int, a)
multipOf x = if length x = 1 then head x else (length x, head x)
gmcompress x = (map multipOf).group $ x
BUT this doesn't work. I think because the then and else clauses have different types, and unfortunately you can't piece-wise define the (co)domain of your functions. How might I go about getting past this issue?
BUT this doesn't work. I think because the then and else clauses have different types, and unfortunately you can't piece-wise define the (co)domain of your functions. How might I go about getting past this issue?
Your diagnosis is right; the then and else must have the same type. There's no "getting past this issue," strictly speaking. Whatever solution you adopt has to use same type in both branches of the conditional. One way would be to design a custom data type that encodes the possibilities that you want, and use that instead. Something like this would work:
-- | A 'Run' of #a# is either 'One' #a# or 'Many' of them (with the number
-- as an argument to the 'Many' constructor).
data Run a = One a | Many Int a
But to tell you the truth, I don't think this would really gain you anything. I'd stick to the (Int, a) encoding rather than going to this Run type.
Related
I am trying to learn haskell and saw a exercise which says
Write two different Haskell functions having the same type:
[a] -> [b] -> Int -> (a,b)
So from my understanding the expressions should take in two lists, an int and return a tuple of the type of the lists.
What i tried so far was
together :: [a] -> [b] -> Int -> (a,b)
together [] [] 0 = (0,0)
together [b] [a] x = if x == a | b then (b,a) else (0,0)
I know I am way off but any help is appreciated!
First you need to make your mind up what the function should return. That is partly determined by the signature. But still you can come up with a lot of functions that return different things, but have the same signature.
Here one of the most straightforward functions is probably to return the elements that are placed on the index determined by the third parameter.
It makes no sense to return (0,0), since a and b are not per se numerical types. Furthermore if x == a | b is not semantically valid. You can write this as x == a || x == b, but this will not work, since a and b are not per se Ints.
We can implement a function that returns the heads of the two lists in case the index is 0. In case the index is negative, or at least one of the two lists is exhausted, then we can raise an error. I leave it as an exercise what to do in case the index is greater than 0:
together :: [a] -> [b] -> Int -> (a,b)
together [] _ = error "List exhausted"
together _ [] = error "List exhausted"
together (a:_) (b:_) 0 = (a, b)
together (a:_) (b:_) n | n < 0 = error "Negative index!"
| …
you thus still need to fill in the ….
I generally dislike those "write any function with this signature"-type excercises precisely because of how arbitrary they are. You're supposed to figure out a definition that would make sense for that particular signature and implement it. In a lot of cases, you can wing it by ignoring as many arguments as possible:
fa :: [a] -> [b] -> Int -> (a,b)
fa (a:_) (b:_) _ = (a,b)
fa _ _ _ = error "Unfortunately, this function can't be made total because lists can be empty"
The error here is the important bit to note. You attempted to go around that problem by returning 0s, but this will only work when 0 is a valid value for types of a and b. Your next idea could be some sort of a "Default" value, but not every type has such a concept. The key observation is that without any knowledge about a type, in order to produce a value from a function, you need to get this value from somewhere else first*.
If you actually wanted a more sensible definition, you'd need to think up a use for that Int parameter; maybe it's the nth element from each
list? With the help of take :: Int -> [a] -> [a] and head :: [a] -> a this should be doable as an excercise.
Again, your idea of comparing x with a won't work for all types; not every type is comparable with an Int. You might think that this would make generic functions awfully limited; that's the point where you typically learn about how to express certain expectations about the types you get, which will allow you to operate only on certain subsets of all possible types.
* That's also the reason why id :: a -> a has only one possible implementation.
Write two different Haskell functions having the same type:
[a] -> [b] -> Int -> (a,b)
As Willem and Bartek have pointed out, there's a lot of gibberish functions that have this type.
Bartek took the approach of picking two based on what the simplest functions with that type could look like. One was a function that did nothing but throw an error. And one was picking the first element of each list, hoping they were not empty and failing otherwise. This is a somewhat theoretical approach, since you probably don't ever want to use those functions in practice.
Willem took the approach of suggesting an actually useful function with that type and proceeded to explore how to exhaust the possible patterns of such a function: For lists, match the empty list [] and the non-empty list a:_, and for integers, match some stopping point, 0 and some categories n < 0 and ….
A question that arises to me is if there is any other equally useful function with this type signature, or if a second function would necessarily have to be hypothetically constructed. It would seem natural that the Int argument has some relation to the positions of elements in [a] and [b], since they are also integers, especially because a pair of single (a,b) is returned.
But the only remotely useful functions (in the sense of not being completely silly) that I can think of are small variations of this: For example, the Int could be the position from the end rather than from the beginning, or if there's not enough elements in one of the lists, it could default to the last element of a list rather than an error. Neither of these are very pleasing to make ("from the end" conflicts with the list being potentially infinite, and having a fall-back to the last element of a list conflicts with the fact that lists don't necessarily have a last element), so it is tempting to go with Bartek's approach of writing the simplest useless function as the second one.
I've been reading some Haskell code and keep seeing functions that look something like this:
ok :: a -> Result i w e a
ok a =
Result $ \i w _ good ->
good i w a
Why is a lambda used? Why wouldn't you just write the following?:
ok :: a -> Result i w e a
ok a =
Result $ good i w a
This is continuation passing style or "CPS".
So first off, your alternative example doesn't make sense. good, i, and w are not known at the point they are used, and you will get an error.
The basic idea of continuation passing style is that instead of returning the relevant information, you instead call a function that you are given (in this case good), passing it your intended result as an argument. Presumably (based on the naming) the ignored argument _ would have been called bad, and it is a function that you would call in the case of failure.
If you are the ok function, it's like the difference between asking you to
Bake me a batch of cookies.
(where I have the intention of giving the cookies to Dave), and
Bake a batch of cookies and then give it to Dave.
which accomplishes the same thing but now I don't have to be a middleman anymore. There are often performance advantages to cutting me out as a middleman, and also it means you can do more things, for example if the batch of cookies is really good you might decide to give it to your mom instead of Dave (thus aborting whatever Dave would have done with them), or bake two batches and give them both to Dave (duplicating what Dave would have done). Sometimes you want this ability and other times you don't, it depends on context. (N.B. in the below examples the types are sufficiently general to disallow these possibilities)
Here is a very simple example of continuation passing style. Say you have a program
pred :: Integer -> Maybe Integer
pred n = if n > 0 then Just (n-1) else Nothing
which subtracts 1 from a number and returns it (in a Just constructor), unless it would become negative then it returns Nothing. You might use it like this:
main = do
x <- readLn
case x of
Just predx -> putStrLn $ "The predecessor is " ++ show predx
Nothing -> putStrLn "Can't take the predecessor"
We can encode this in continuation passing style by, instead of returning Maybe, have pred take an argument for what to do in each case:
pred :: Integer -> (Integer -> r) -> r -> r
-- ^^^^^^^^^^^^^^ ^
-- Just case |
-- Nothing case
pred n ifPositive ifNegative =
if n > 0
then ifPositive (n-1)
else ifNegative
And the usage becomes:
main = do
x <- readLn
pred x (\predx -> putStrLn $ "The predecessor is " ++ show predx)
(putStrLn "Can't take the predecessor)
See how that works? -- doing it the first way we got the result and then did case analysis; in the second way each case became an argument to the function. And in the process the call to pred became a tail call, eliminating the need for a stack frame and the intermediate Maybe data structure.
The only remaining problem is that the pred's signature is kind of confusing. We can make it a bit clearer by wrapping the CPS stuff in its own type constructor:
newtype CPSMaybe a = CPSMaybe (forall r. (a -> r) -> r -> r)
pred :: Integer -> CPSMaybe Integer
pred n = CPSMaybe $ \ifPositive ifNegative ->
if n > 0
then ifPositive (n-1)
else ifNegative
which has a signature that looks more like the first one but with code that looks like the second (except for the CPSMaybe newtype wrapper, which has no effect at runtime). And now maybe you can see the connection to the code in your question.
Well, the Result type apparently wraps a function, so a lambda is the natural thing to use here. If you wanted to avoid a lambda, you could use a local definition instead using let or where, e.g.:
ok a = let
proceed i w _ good = good i w a
in Result proceed
-- or --
ok a = Result proceed
where
proceed i w _ good = good i w a
Writing this won’t work because the variables i, w, and good are not in scope:
ok :: a -> Result i w e a
ok a =
Result $ good i w a
I wonder if the source of your confusion is the fact that i and w are also used as type variables in the signature of ok, but they’re different variables that happen to have the same names. It’s just as if you’d written something like this:
ok :: a -> Result i w e a
ok value =
Result $ continue index writer value
Here it should be obvious that the continue, index, and writer variables aren’t defined.
In the first sample, good, w and i are locally defined parameters to the lambda expression. In the second sample, they are free variables. I would expect the second sample to fail with an error saying that those identifiers are not in scope. Result apparently is a type that contains information about how to use given data and handlers. ok says to take the data and apply the handler indicating a good outcome to it. In the second sample, it is not clear that one is even refering to the arguments available to what Result wraps, or which names refer to which arguments.
I was playing around Maybe and Either monad types (Chaining, applying conditional functions according to returned value, also returning error message which chained function has failed etc.). So it seemes to me like we can achieve same and more things that Maybe does by using Either monad. So my question is where the practical or conceptual difference between those ?
You are of course right that Maybe a is isomorphic to Either Unit a. The thing is that they are often semantically used to denote different things, a bit like the difference between returning null and throwing a NoSuchElementException:
Nothing/None denotes the "expected" missing of something, while
Left e denotes an error in getting it, for whatever reason.
That said, we might even combine the two to something like:
query :: Either DBError (Maybe String)
where we express both the possibility of a missing value (a DB NULL) and an error in the connection, the DBMS, or whatever (not saying that there aren't better designs, but you get the point).
Sometimes, the border is fluid; for saveHead :: [a] -> Maybe a, we could say that the expected possibility of the error is encoded in the intent of the function, while something like saveDivide might be encoded as Float -> Float -> Either FPError Float or Float -> Float -> Maybe Float, depending on the use case (again, just some stupid examples...).
If in doubt, the best option is probably to use a custom result ADT with semantic encoding (like data QueryResult = Success String | Null | Failure DBError), and to prefer Maybe to simple cases where it is "traditionally expected" (a subjective point, which however will be mostly OK if you gain experience).
#phg's answer is great. I will chime in with something that helped clear it up for me when I was learning them:
Maybe is one (value) or none – ie, you have a value or you have nothing
Either is a logical disjunction, but you always have at least one (value) - ie, you have one or the other, but not both.
Maybe is great for things like where you may or may not have a value - for example looking for an item in a list. if the list contains it, we get (Just x) otherwise we get Nothing
Either is the perfect representation of a branch in your code - it's going to go one way or the other; Left or Right. We use a mnemonic to remember it: Right is the right (correct) way; Left is the wrong way (Error). This is not it's only use of course, but definitely the most common.
I know the differences might seem subtle at first, but really they're suitable for very different things.
Well, you see, we can put this to the extreme by saying that all product types can be represented by just 2-tuples and all non-recursive sum types by Either. To additionally represent recursive types, we need a fixpoint type.
For example, why have 4-tuples (a,b,c,d) when we could as well write (a, (b, (c,d))) or (((a,b), c), d) ?
Or why have lists, when the following works as well?
data Y f = Y (f (Y f))
type List a = Y ((,) (Either () a))
nil = Y (Left (), undefined)
cons a as = Y (Right a, as)
infixr 4 cons
numbers = 1 `cons` 2 `cons` 3 `cons` nil
-- this is like foldl
reduce f z (Y (Left (), _)) = z
reduce f z (Y (Right x, xs)) = reduce f (f z x) xs
total = reduce (+) 0 numbers
I've just started trying to learn haskell and functional programming. I'm trying to write this function that will convert a binary string into its decimal equivalent. Please could someone point out why I am constantly getting the error:
"BinToDecimal.hs":19 - Syntax error in expression (unexpected `}', possibly due to bad layout)
module BinToDecimal where
total :: [Integer]
total = []
binToDecimal :: String -> Integer
binToDecimal a = if (null a) then (sum total)
else if (head a == "0") then binToDecimal (tail a)
else if (head a == "1") then total ++ (2^((length a)-1))
binToDecimal (tail a)
So, total may not be doing what you think it is. total isn't a mutable variable that you're changing, it will always be the empty list []. I think your function should include another parameter for the list you're building up. I would implement this by having binToDecimal call a helper function with the starting case of an empty list, like so:
binToDecimal :: String -> Integer
binToDecimal s = binToDecimal' s []
binToDecimal' :: String -> [Integer] -> Integer
-- implement binToDecimal' here
In addition to what #Sibi has said, I would highly recommend using pattern matching rather than nested if-else. For example, I'd implement the base case of binToDecimal' like so:
binToDecimal' :: String -> [Integer] -> Integer
binToDecimal' "" total = sum total -- when the first argument is the empty string, just sum total. Equivalent to `if (null a) then (sum total)`
-- Include other pattern matching statements here to handle your other if/else cases
If you think it'd be helpful, I can provide the full implementation of this function instead of giving tips.
Ok, let me give you hints to get you started:
You cannot do head a == "0" because "0" is String. Since the type of a is [Char], the type of head a is Char and you have to compare it with an Char. You can solve it using head a == '0'. Note that "0" and '0' are different.
Similarly, rectify your type error in head a == "1"
This won't typecheck: total ++ (2^((length a)-1)) because the type of total is [Integer] and the type of (2^((length a)-1)) is Integer. For the function ++ to typecheck both arguments passed to it should be list of the same type.
You are possible missing an else block at last. (before the code binToDecimal (tail a))
That being said, instead of using nested if else expression, try to use guards as they will increase the readability greatly.
There are many things we can improve here (but no worries, this is perfectly normal in the beginning, there is so much to learn when we start Haskell!!!).
First of all, a string is definitely not an appropriate way to represent a binary, because nothing prevents us to write "éaldkgjasdg" in place of a proper binary. So, the first thing is to define our binary type:
data Binary = Zero | One deriving (Show)
We just say that it can be Zero or One. The deriving (Show) will allow us to have the result displayed when run in GHCI.
In Haskell to solve problem we tend to start with a more general case to dive then in our particular case. The thing we need here is a function with an additional argument which holds the total. Note the use of pattern matching instead of ifs which makes the function easier to read.
binToDecimalAcc :: [Binary] -> Integer -> Integer
binToDecimalAcc [] acc = acc
binToDecimalAcc (Zero:xs) acc = binToDecimalAcc xs acc
binToDecimalAcc (One:xs) acc = binToDecimalAcc xs $ acc + 2^(length xs)
Finally, since we want only to have to pass a single parameter we define or specific function where the acc value is 0:
binToDecimal :: [Binary] -> Integer
binToDecimal binaries = binToDecimalAcc binaries 0
We can run a test in GHCI:
test1 = binToDecimal [One, Zero, One, Zero, One, Zero]
> 42
OK, all fine, but what if you really need to convert a string to a decimal? Then, we need a function able to convert this string to a binary. The problem as seen above is that not all strings are proper binaries. To handle this, we will need to report some sort of error. The solution I will use here is very common in Haskell: it is to use "Maybe". If the string is correct, it will return "Just result" else it will return "Nothing". Let's see that in practice!
The first function we will write is to convert a char to a binary. As discussed above, Nothing represents an error.
charToBinary :: Char -> Maybe Binary
charToBinary '0' = Just Zero
charToBinary '1' = Just One
charToBinary _ = Nothing
Then, we can write a function for a whole string (which is a list of Char). So [Char] is equivalent to String. I used it here to make clearer that we are dealing with a list.
stringToBinary :: [Char] -> Maybe [Binary]
stringToBinary [] = Just []
stringToBinary chars = mapM charToBinary chars
The function mapM is a kind of variation of map which acts on monads (Maybe is actually a monad). To learn about monads I recommend reading Learn You a Haskell for Great Good!
http://learnyouahaskell.com/a-fistful-of-monads
We can notice once more that if there are any errors, Nothing will be returned.
A dedicated function to convert strings holding binaries can now be written.
binStringToDecimal :: [Char] -> Maybe Integer
binStringToDecimal = fmap binToDecimal . stringToBinary
The use of the "." function allow us to define this function as an equality with another function, so we do not need to mention the parameter (point free notation).
The fmap function allow us to run binToDecimal (which expect a [Binary] as argument) on the return of stringToBinary (which is of type "Maybe [Binary]"). Once again, Learn you a Haskell... is a very good reference to learn more about fmap:
http://learnyouahaskell.com/functors-applicative-functors-and-monoids
Now, we can run a second test:
test2 = binStringToDecimal "101010"
> Just 42
And finally, we can test our error handling system with a mistake in the string:
test3 = binStringToDecimal "102010"
> Nothing
Here's a simple, barebones example of how the code that I'm trying to do would look in C++.
while (state == true) {
a = function1();
b = function2();
state = function3();
}
In the program I'm working on, I have some functions that I need to loop through until bool state equals false (or until one of the variables, let's say variable b, equals 0).
How would this code be done in Haskell? I've searched through here, Google, and even Bing and haven't been able to find any clear, straight forward explanations on how to do repetitive actions with functions.
Any help would be appreciated.
Taking Daniels comment into account, it could look something like this:
f = loop init_a init_b true
where
loop a b True = loop a' b' (fun3 a' b')
where
a' = fun1 ....
b' = fun2 .....
loop a b False = (a,b)
Well, here's a suggestion of how to map the concepts here:
A C++ loop is some form of list operation in Haskell.
One iteration of the loop = handling one element of the list.
Looping until a certain condition becomes true = base case of a function that recurses on a list.
But there is something that is critically different between imperative loops and functional list functions: loops describe how to iterate; higher-order list functions describe the structure of the computation. So for example, map f [a0, a1, ..., an] can be described by this diagram:
[a0, a1, ..., an]
| | |
f f f
| | |
v v v
[f a0, f a1, ..., f an]
Note that this describes how the result is related to the arguments f and [a0, a1, ..., an], not how the iteration is performed step by step.
Likewise, foldr f z [a0, a1, ..., an] corresponds to this:
f a0 (f a1 (... (f an z)))
filter doesn't quite lend itself to diagramming, but it's easy to state many rules that it satisfies:
length (filter pred xs) <= length xs
For every element x of filter pred xs, pred x is True.
If x is an element of filter pred xs, then x is an element of xs
If x is not an element of xs, then x is not an element of filter pred xs
If x appears before x' in filter pred xs, then x appears before x' in xs
If x appears before x' in xs, and both x and x' appear in filter pred xs, then x appears before x' in filter pred xs
In a classic imperative program, all three of these cases are written as loops, and the difference between them comes down to what the loop body does. Functional programming, on the contrary, insists that this sort of structural pattern does not belong in "loop bodies" (the functions f and pred in these examples); rather, these patterns are best abstracted out into higher-order functions like map, foldr and filter. Thus, every time you see one of these list functions you instantly know some important facts about how the arguments and the result are related, without having to read any code; whereas in a typical imperative program, you must read the bodies of loops to figure this stuff out.
So the real answer to your question is that it's impossible to offer an idiomatic translation of an imperative loop into functional terms without knowing what the loop body is doing—what are the preconditions supposed to be before the loop runs, and what the postconditions are supposed to be when the loop finishes. Because that loop body that you only described vaguely is going to determine what the structure of the computation is, and different such structures will call for different higher-order functions in Haskell.
First of all, let's think about a few things.
Does function1 have side effects?
Does function2 have side effects?
Does function3 have side effects?
The answer to all of these is a resoundingly obvious YES, because they take no inputs, and presumably there are circumstances which cause you to go around the while loop more than once (rather than def function3(): return false). Now let's remodel these functions with explicit state.
s = initialState
sentinel = true
while(sentinel):
a,b,s,sentinel = function1(a,b,s,sentinel)
a,b,s,sentinel = function2(a,b,s,sentinel)
a,b,s,sentinel = function3(a,b,s,sentinel)
return a,b,s
Well that's rather ugly. We know absolutely nothing about what inputs each function draws from, nor do we know anything about how these functions might affect the variables a, b, and sentinel, nor "any other state" which I have simply modeled as s.
So let's make a few assumptions. Firstly, I am going to assume that these functions do not directly depend on nor affect in any way the values of a, b, and sentinel. They might, however, change the "other state". So here's what we get:
s = initState
sentinel = true
while (sentinel):
a,s2 = function1(s)
b,s3 = function2(s2)
sentinel,s4 = function(s3)
s = s4
return a,b,s
Notice I've used temporary variables s2, s3, and s4 to indicate the changes that the "other state" goes through. Haskell time. We need a control function to behave like a while loop.
myWhile :: s -- an initial state
-> (s -> (Bool, a, s)) -- given a state, produces a sentinel, a current result, and the next state
-> (a, s) -- the result, plus resultant state
myWhile s f = case f s of
(False, a, s') -> (a, s')
(True, _, s') -> myWhile s' f
Now how would one use such a function? Well, given we have the functions:
function1 :: MyState -> (AType, MyState)
function2 :: MyState -> (BType, MyState)
function3 :: MyState -> (Bool, MyState)
We would construct the desired code as follows:
thatCodeBlockWeAreTryingToSimulate :: MyState -> ((AType, BType), MyState)
thatCodeBlockWeAreTryingToSimulate initState = myWhile initState f
where f :: MyState -> (Bool, (AType, BType), MyState)
f s = let (a, s2) = function1 s
(b, s3) = function2 s2
(sentinel, s4) = function3 s3
in (sentinel, (a, b), s4)
Notice how similar this is to the non-ugly python-like code given above.
You can verify that the code I have presented is well-typed by adding function1 = undefined etc for the three functions, as well as the following at the top of the file:
{-# LANGUAGE EmptyDataDecls #-}
data MyState
data AType
data BType
So the takeaway message is this: in Haskell, you must explicitly model the changes in state. You can use the "State Monad" to make things a little prettier, but you should first understand the idea of passing state around.
Lets take a look at your C++ loop:
while (state == true) {
a = function1();
b = function2();
state = function3();
}
Haskell is a pure functional language, so it won't fight us as much (and the resulting code will be more useful, both in itself and as an exercise to learn Haskell) if we try to do this without side effects, and without using monads to make it look like we're using side effects either.
Lets start with this structure
while (state == true) {
<<do stuff that updates state>>
}
In Haskell we're obviously not going to be checking a variable against true as the loop condition, because it can't change its value[1] and we'd either evaluate the loop body forever or never. So instead, we'll want to be evaluating a function that returns a boolean value on some argument:
while (check something == True) {
<<do stuff that updates state>>
}
Well, now we don't have a state variable, so that "do stuff that updates state" is looking pretty pointless. And we don't have a something to pass to check. Lets think about this a bit more. We want the something to be checked to depend on what the "do stuff" bit is doing. We don't have side effects, so that means something has to be (or be derived from) returned from the "do stuff". "do stuff" also needs to take something that varies as an argument, or it'll just keep returning the same thing forever, which is also pointless. We also need to return a value out all this, otherwise we're just burning CPU cycles (again, with no side effects there's no point running a function if we don't use its output in some way, and there's even less point running a function repeatedly if we never use its output).
So how about something like this:
while check func state =
let next_state = func state in
if check next_state
then while check func next_state
else next_state
Lets try it in GHCi:
*Main> while (<20) (+1) 0
20
This is the result of applying (+1) repeatedly while the result is less than 20, starting from 0.
*Main> while ((<20) . length) (++ "zob") ""
"zobzobzobzobzobzobzob"
This is the result of concatenating "zob" repeatedly while the result's length is less than 20, starting from the empty string.
So you can see I've defined a function that is (sort of a bit) analogous to a while loop from imperative languages. We didn't even need dedicated loop syntax for it! (which is the real reason Haskell has no such syntax; if you need this kind of thing you can express it as a function). It's not the only way to do so, and experienced Haskell programmers would probably use other standard library functions to do this kind of job, rather than writing while.
But I think it's useful to see how you can express this kind of thing in Haskell. It does show that you can't translate things like imperative loops directly into Haskell; I didn't end up translating your loop in terms of my while because it ends up pretty pointless; you never use the result of function1 or function2, they're called with no arguments so they'd always return the same thing in every iteration, and function3 likewise always returns the same thing, and can only return true or false to either cause while to keep looping or stop, with no information resulting.
Presumably in the C++ program they're all using side effects to actually get some work done. If they operate on in-memory things then you need to translate a bigger chunk of your program at once to Haskell for the translation of this loop to make any sense. If those functions are doing IO then you'll need to do this in the IO monad in Haskell, for which my while function doesn't work, but you can do something similar.
[1] As an aside, it's worth trying to understand that "you can't change variables" in Haskell isn't just an arbitrary restriction, nor is it just an acceptable trade off for the benefits of purity, it is a concept that doesn't make sense the way Haskell wants you to think about Haskell code. You're writing down expressions that result from evaluating functions on certain arguments: in f x = x + 1 you're saying that f x is x + 1. If you really think of it that way rather than thinking "f takes x, then adds one to it, then returns the result" then the concept of "having side effects" doesn't even apply; how could something existing and being equal to something else somehow change a variable, or have some other side effect?
You should write a solution to your problem in a more functional approach.
However, some code in haskell works a lot like imperative looping, take for example state monads, terminal recursivity, until, foldr, etc.
A simple example is the factorial. In C, you would write a loop where in haskell you can for example write fact n = foldr (*) 1 [2..n].
If you've two functions f :: a -> b and g :: b -> c where a, b, and c are types like String or [Int] then you can compose them simply by writing f . b.
If you wish them to loop over a list or vector you could write map (f . g) or V.map (f . g), assuming you've done Import qualified Data.Vector as V.
Example : I wish to print a list of markdown headings like ## <number>. <heading> ## but I need roman numerals numbered from 1 and my list headings has type type [(String,Double)] where the Double is irrelevant.
Import Data.List
Import Text.Numeral.Roman
let fun = zipWith (\a b -> a ++ ". " ++ b ++ "##\n") (map toRoman [1..]) . map fst
fun [("Foo",3.5),("Bar",7.1)]
What the hell does this do?
toRoman turns a number into a string containing the roman numeral. map toRoman does this to every element of a loop. map toRoman [1..] does it to every element of the lazy infinite list [1,2,3,4,..], yielding a lazy infinite list of roman numeral strings
fst :: (a,b) -> a simply extracts the first element of a tuple. map fst throws away our silly Meow information along the entire list.
\a b -> "##" ++ show a ++ ". " ++ b ++ "##" is a lambda expression that takes two strings and concatenates them together within the desired formatting strings.
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] takes a two argument function like our lambda expression and feeds it pairs of elements from it's own second and third arguments.
You'll observe that zip, zipWith, etc. only read as much of the lazy infinite list of Roman numerals as needed for the list of headings, meaning I've number my headings without maintaining any counter variable.
Finally, I have declared fun without naming it's argument because the compiler can figure it out from the fact that map fst requires one argument. You'll notice that put a . before my second map too. I could've written (map fst h) or $ map fst h instead if I'd written fun h = ..., but leaving the argument off fun meant I needed to compose it with zipWith after applying zipWith to two arguments of the three arguments zipWith wants.
I'd hope the compiler combines the zipWith and maps into one single loop via inlining.