list,haskell,io , Mapping with IO actions in Haskell

Mapping with IO actions in Haskell


Tag: list,haskell,io

I have a function that takes two filenames, and reads the contents of those two files into Strings, and then returns if they match or not. Here's the function:

f :: String -> String -> IO Bool
f fileName1 fileName2 = do
        str1 <- readFile fileName1
        str2 <- readFile fileName2
        return (str1 == str2)

And if I use it like this from inside main:

main = do
        res <- f "A.txt" "B.txt"
        print res

It works and prints either True or False. What I want to do is, apply this function f to a list (of tuples) of filenames. For something like:

[("a.txt", "b.txt"), ("c.txt", "d.txt")]

(Assume that a.txt and b.txt have the same content and c.txt and d.txt are different).

I want to transform it (the list of file names) into a Bool list like: [True, False]. I tried using mapM, but that doesn't appear to map anything (when I print the list after using mapM, it prints the same list of tuples).

So my question is: What am I doing wrong, and how can I get a list of Bools like I mentioned above?

Please go easy on me as I'm still quite new to Haskell and functional programming :)


Here is a function f' which does what you describe.

f' :: [(String,String)] -> IO [Bool]
f' = mapM $ uncurry f

Let me know if something is unclear! And, just to be clear, here is how you run it:

main = do
        res <- f' [("a.txt", "b.txt"), ("c.txt", "d.txt")]
        print res


The function is in pointfree form, so it is equivalent to f' lst = mapM (uncurry f) lst. mapM essentially maps each element of lst using f as the function, and pushes the IO to the outside of the list.

uncurry just takes a function of the form a -> b -> c and transforms it into one (a,b) -> c, which is what we want since you have a list of tuples.


Haskell - generate and use the same random list

In Haskell, I'd like to generate a random list of Ints and use it throughout my program. My current solution causes the array to be created randomly each time I access/use it. How can I overcome this problem?...

group indices of list in list of lists

I am looking for an elegant solution for the following problem. I have a list of ints and I want to create a list of lists where the indices with the same value are grouped together in the order of the occurrences of said list. [2, 0, 1, 1, 3,...

chunk of data into fixed lengths chunks and then add a space and again add them all as a string

I have got hex values as a85b080040010000. I want it to be as a8 5b 08 00 40 01 00 00. I have done it by using below code. But I have to work with very large data. So I want computed time to be very low. import binascii import...

apply a transformation with function inline

Starting from a simple case of "fold" (I used (+) but can be anything else): Prelude.foldl (+) 0 [10,20,30] is it possible apply an inline transformation similar to (that doesn't work): Prelude.foldl ((+) . (\x -> read x :: Int)) 0 ["10","20","30"] In case not, is there an alternative to...

Sort List of Numbers according to Custom Number Sequence

Question :A set of numbers will be passed as input. Also the redefined relationship of the digits 0-9 in ascending order will be passed as input. Based on the redefined relationship, the set of numbers must be listed in ascending order. Input Format: The first line will contain the the...

Python regular expression, matching the last word

I've the following problem. I'm looking to find all words in a string that typically looks like so HelloWorldToYou Notice, each word is capitalized as a start followed by the next word and so on. I'm looking to create a list of words from it. So the final expected output...

Saving elements of a list as data.frames using R

How can I save each element of a list in a in a separate .RData file? Consider the following example: # Generating a list containing 3 matrices set.seed(1) mylist=list(M1=matrix(LETTERS[sample(1:26,9)],3),M2=matrix(LETTERS[sample(1:26,9)],3),M3=matrix(LETTERS[sample(1:26,9)],3)) mylist[1:2] # $M1 # [,1] [,2] [,3] # [1,] "G" "U" "W" # [2,] "J" "E" "M" # [3,] "N" "S"...

Reduction of list dimensions in Python

I'm trying to assign classes to a list of nodes, and separate all nodes into separate lists based on class tag. For example, if we have the following code: #define number of classes MaxC=5 index=[4 4 5 1 4 1 4 5 4 4 3 1 3 3 1 1]...

Haskell do clause with multiple monad types

I'm using a graphic library in Haskell called ThreePennyUI. In this library the main function returns a UI monad object. This causes me much headache as when I attempt to unpack IO values into local variables I receive errors complaining of different monad types. Here's an example of my problem:...

Prolog: Summing elements of two lists representing an integer(restrictions inside not regular sum!!)

I was solving a problem: A list is representing an integer say 12345 by L=[12,34,5] each element should be from 0 to 99.The exercise is to write a function (sum) that sums two lists and gives the equivalent list of their sum which is representing the sum of two integers....

Zip with tuples and list

I need to write a code that will return the characters of two strings as a list of tuples. The code should stop when either one of the strings runs out of characters. I know is a simple code but I cannot get to work where it gives me more...

How can I iterate through nested HTML lists without returning the “youngest” children?

Fiddle: I want to iterate through an HTML nested list that is 3-layers deep. <ul> <li>animals <ul> <li>birds <ul> <li>crow</li> <li>parrot</li> </ul> </li> <li>reptiles</li> </ul> </li> <li>plants</li> <li>bugs</li> </ul> I want it to iterate through the list so that it returns the elements in this order (note, this isn't...

Haskell: When declaring a class, how can I use a type variable that is not immediately in the constructors?

I want to define a function, <-? to check whether an element is in a list/set/map. module Test where import qualified Data.Map as Map import qualified Data.Set as Set class Memberable a where (<-?) :: b -> a -> Bool instance Memberable [x] where (<-?) = elem instance Memberable (Map.Map...

I need to make sure that only certain characters are in a list?

I have this to get input and put it in a list: def start(): move_order=[raw_input("Enter your moves: ").split()] And I only want the characters A, D, S, C, H (it's for a game >_>) to be allowed. I've tried using the regular expressions stuff: if re.match('[ADSCH]+', [move_order]) is False: print...

Replace all [ ] with {} - as short as possible [on hold]

Given the code below: import Data.List; main = (readLn :: IO [Integer]) >>= print . subsequences It takes a list of integers from standard input (for example [1,2,3]) and outputs something like: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]] I want it to be like this: {},{1},{2},{1,2},{3},{1,3},{2,3},{1,2,3}} so my goal is to replace every [ and...

Get element starting with letter from List

I have a list and I want to get the position of the string which starts with specific letter. I am trying this code, but it isn't working. List<String> sp = Arrays.asList(splited); int i2 = sp.indexOf("^w.*$"); ...

Python - Using a created list as a parameter

When I run my code it tells me: Type Error: unorderable types: str() < float(). I can't figure out why it won't let me compare these two numbers. The list I am using is defined, and the numbers in it have been redefined as floats, so I'm not sure what...

Operand order in Scala List.prepend (::)

Odersky has brilliantly optimized Java syntax, enabling object calls without dots and parenthesis. I.e. instead of list.prepend(item), you now simply write list :: item, which also turns language operators into simple object methods. Here, List defines :: (prepend) operator. However, you normally write it vice-verse in Scala, using item ::...

Filter list using Boolean array

How can I use boolean array as index to filter a list? For example: >>> l = ['a','b','c'] >>> b = [True,False,False] >>> l[b] The result should be: ['a'] I know numpy support it but want to know how to solve in Python. >>> import numpy as np >>> l...

Create array/list of many objects(initially unknown amount) by tag

I'm currently working on a radar system for my space game, and I am trying to work out how to add gameobjects, by tag, to either a list or array that can then be used in other methods. I can't do this manually because I will be procedurally generating each...

Combining Event and an attribute in threepenny-gui

I have an Event String which I want to sink into a textarea. This works fine, but now I want to combine the current value of a checkbox selection to this string. First I did this by using checkedChange of the checkbox which works, but has a problem. If the...

Haskell IO - read from standard input directly to list

Haskell IO system is super hard to understand for me so i have question : How to read from standard input to list ? I know that there is function getLine :: IO String and interact. But i do not know how to convert the input to list so I...

Stopping condition on a recursive function - Haskell

So, I have this function which aims to align the text on the left without cutting words(only white spaces). However my problem is that I cannot find a stopping condition of the function and it goes infinitely. f n "" = "" --weak condition f n s = if n...

Decremented value called in the recursion in Haskell

So, I have this function that aligns the text to the left for a given column width. I have already fixed some of its problems, but I am unable to make it not use the decremented parameter once it is called, but to return to the starting value of n....

represent an index inside a list as x,y in python

I have a list which contains 1000 integers. The 1000 integers represent 20X50 elements of dimensional array which I read from a file into the list. I need to walk through the list with an indicator in order to find close elements to each other. I want that my indicator...

Insertion into a list doesn't reflect outside function whereas deletion does?

I am new to Lisp. Deletion of an item in a list by a function gets reflected outside the function but insertion doesn't. How can I do the same for insertion? For example (defun test (a b) (delete 1 a) (delete 5 b) (append '(5) b) (member '5 b)) (setq...

Keep track of loop without a counter

Say, I got a list which length can be odd or even. Every iteration , I remove two items from the list. If there is one or no item at the end, I end the execution. If I store (length list)/2 every loop, I will get e.g. [5,4,3...] for a...

Implementing a dictionary function to calculate the average of a list

As always, I've attempted this for awhile before I proceed to ask a question on here. I know there are several attempts at answering this, but none really worked for what I needed. Here are the instructions: Implement the following three functions (you should use an appropriate looping construct to...

Loop by Object inside another Object in Java

I'm not sure how to ask this in word so here's an example : Building class have a data like this : Map<Season, List<Info>> infosBySeason = new TreeMap<Season, List<Info>>(); What I'd like to do is loop for each Building, print by Season infos. Like : Season 1 : Building 1...

How do I read this list and parse it?

I'm using requests and the output I get from the sites API is a list, I've been stuck trying to parse it to get the data from it. I use r = requests.get(urlas, params=params) r.json() to get the data I want. Here is a snippet of the list [{'relation_type': None,...

Update list of items in c#

I would like to know if you can suggest me an efficient way to update a list of items in c#. Here is a generic example: If CurrentList is [ {Id: 154, Name: "George", Salary: 10 000} {Id: 233, Name: "Alice", Salary: 10 000}] And NewList is [ {Id: 154,...

Stopping list selection in Python 2.7

Imagine that I have an order list of tuples: s = [(0,-1), (1,0), (2,-1), (3,0), (4,0), (5,-1), (6,0), (7,-1)] Given a parameter X, I want to select all the tuples that have a first element equal or greater than X up to but not including the first tuple that has...

How can I express foldr in terms of foldMap for type-aligned sequences?

I'm playing around with type-aligned sequences, and in particular I'm messing around with the idea of folding them. A foldable type-aligned sequence looks something like this: class FoldableTA fm where foldMapTA :: Category h => (forall b c . a b c -> h b c) -> fm a b...

Django: Handling several page parameters

I have several possible parameter to process in a page. Assume x0, x1, x2,..., x1000. It seems awkward to get and process them one by one by request.GET.get('x0'), request.GET.get('x1'), ... Any idea to put them in a list, so that they can be processed in a loop....

How to use XDocument to get attributes and add them to a List

I am trying to load data from an XML file and add it to a List. The XML file looks like this: and this is my code: public void LoadPayments(List<List<string>> list1, List<List<string>> list2) { try { if (File.Exists(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "RentData.xml"))) { int count = 0; XDocument doc; using (var reader =...

why java API prevents us to call add and remove together?

As per Java API- IllegalStateException - if neither next nor previous have been called, or remove or add have been called after the last call to next or previous remove()- Removes from the list the last element that was returned by next() or previous() (optional operation). This call can only...

Java Get and then remove from a list

In my code i have a list of instances of a class. And i want to get a attribute of 1 instance which is ArrayList. In this class i have implement getters and setters. So I call listofinstances.get(i).getArrayList().remove(0); in order to remove the 1st item of this list. Is this...

Python 3.4: List to Dictionary

I have a string as follows : ['Total Revenue', 31821000, 30871000, 29904000, 'Cost of Revenue', 16447000, 16106000, 15685000, 'Gross Profit', 15374000, 14765000, 14219000, 'Research Development', 1770000, 1715000, 1634000, 'Selling General and Administrative', 6469000, 6384000, 6102000, 'Non Recurring', '-', '-', '-', 'Others', '-', '-', '-', 'Total Operating Expenses', '-', '-', '-',...

Sort when values are None or empty strings python

I have a list with dictionaries in which I sort them on different values. I'm doing it with these lines of code: def orderBy(self, col, dir, objlist): if dir == 'asc': sorted_objects = sorted(objlist, key=lambda k: k[col]) else: sorted_objects = sorted(objlist, key=lambda k: k[col], reverse=True) return sorted_objects Now the problem...

how to insert into python nested list

I want to insert an item into a list inside a list. I'm wondering if someone can show me. list5 = [[], [(1,2,3,4), 2, 5]] print("1. list5", list5) list5.insert(0, (2,5,6,8)) print("2. list5", list5) Output: 1. list5 [[], [(1, 2, 3, 4), 2, 5]] 2. list5 [(2, 5, 6, 8), [],...

Implementing map on a tree using fold

I am trying to implement a map using fold. I could do so in Haskell data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show) foldTree :: Tree a -> b -> (b -> a -> b -> b) -> b foldTree EmptyTree d _ =...

First three items of a list in Haskell

I am very new to Haskell, and struggling a bit with a function here. The premise is simple enough: Run through a list, and combine each 3 items next to each other with another function and return a list with the results. The problem is to do it in a...

Python RuntimeError: maximum recursion depth exceeded in cmp

I have a complex data structure that I'm trying to process. Explanation of the data structure: I have a dictionary of classes. The key is a name. The value is a class reference. The class contains two lists of dictionaries. Here's a simple example of my data structure: import scipy.stats...

List of tuples from (a, all b) to (b, all a)

I am starting with a list of tuples (a,all b). I want to end with a list of tuples (b,all a). For example: FROM (a1,[b1,b2,b3]) (a2,[b2]) (a3,[b1,b2]) TO (b1,[a1,a3]) (b2[a1,a2,a3]) (b3,[a1] How do I do this using Python 2? Thank you for your help....

ZipList with Scalaz

Suppose I have a list of numbers and list of functions to apply to numbers: val xs: List[Int] = List(1, 2, 3) val fs: List[Int => Int] = List(f1, f2, f3) Now I would like to use an Applicative to apply f1 to 1, f2 to 2, etc. val ys:...

Easiest way to Add lines wrong a .txt file to a list

At the moment I am opening the .txt file twice, once to get the number of all the lines, second to add a line to a list as much as how much lines there are in the .txt file. Is there an easier/better way to do this? This is my...

join two different list by id into one list

I've got two different list of two different objects. Then i got one list of a viewmodel that contains properties from both the objects and i want them to be joined into that list. //Product public string id { get; set; } public string unitMeasurement { get; set; } public...

Python 2.7 “list index out of range”

I keep getting "IndexError: list index out of range", the code does fine with things like "s = 'miruxsexxzlbveznyaidekl'" but this particular length makes it throw an error. Can anyone help me understand what I did wrong here, not just give me the answer? (I'd like to not have to...

Can I put StreamReaders in a list? Or any other way to read a lot of text files at once?

I have a lot of text files and want to read them all by once, how do I do this? This is my code till now: List<StreamReader> lijst = new List<StreamReader>(); using (StreamReader qwe = new StreamReader("C:\\123.txt")) using (StreamReader qwer = new StreamReader("C:\\1234.txt")) lijst.Add(qwe); lijst.Add(qwer); But I get an ObjectDisposedException(Cannot...

Why cant I refer to a random index in my 4D list, while I know it exists?

I got a 4D list, and I want where I want to display only the [k][3][j][z], but this isnt working. I checked all the counts and they are all 5+, so 3[4] should work... for (int k = 0; k < lijst4D.Count; k++) { for (int i = 0; i...