Questions tagged [monads]

A monad in programming is a composable computation description. Monads are an important construct in functional programming languages like Haskell.

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Typescript Maybe functor

I have been trying to implement a version of the Maybe functor in Typescript. I'm looking for feedback on whether I can improve the type safety of my implementation. I'm not particularly happy that ...
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Count number of days within each date range in a list of date ranges

Happily picked up Haskell a couple days back and working on the following use case. Given a list of JSON objects with date fields and a start date, I want to create a list of weeks (here a tuple of ...
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Hamming distance between two strings

I wrote this module to find the Hamming distance between two strings. (It's a problem from exercism.io's Haskell track.) As I saw it, the problem has two distinct ...
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160 views

A beginner's brainfuck interpreter in Haskell

I am rather new to haskell, and could use some feedback on my code and the decisions I had to make. In my previous project, I made a JSON parser, but relied heavily on guidance from a university ...
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Haskell - saving content of HTTP request in selected file

As a Haskell beginner I'm currently playing around the IO monad and prepared simple program which gets from the user two information: URL (http) Output file path Then it performs ...
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Either monad in C++11

I'm using C++11 with https://github.com/SmingHub/Sming. It has no support of std lib or exceptions. I wanted to use Either monad ...
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281 views

Telegram bot in Haskell using custom monad transformers

Note: I show almost all of my code for completeness, but I really only want the review to focus on Session.hs, Handler.hs, and maybe Controller.hs. I can delete the extra code from the review or ...
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1answer
217 views

How can I use the State monad effectively in Haskell?

I have written a simple fibonacci function in Haskell that uses the State monad to save solutions to overlapping sub-problems. Now despite the function working I am not really happy with the solution, ...
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Loading Files from a Directory in Haskell

I'm a very new Haskell programmer, and I've written the following code for loading TSV files. I think it can be improved, but I'm not sure how. I feel like the giant "do" block in loadData is ...
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86 views

Monad to sample without replacement

I created a monad in Haskell that lets you sample without replacement from user-defined urns, and then at the end gives you a list of all possible outcomes. It looks like it's similar to the list ...
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75 views

Typescript monad for data being loaded

I have been struggling to make typescript happy about the types. I am pulling data from our backend API and I wanted to give context to the data. Basically it is a monad with 4 shapes: Initial (...
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181 views

C++17 Either implementation for error handling [closed]

This is a C++17 Either implementation for error handling. First intent is I want to improve myself. Second intent is I want to create a more expressive solution than variant for exception free error ...
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Implementation of partitionM

I needed a monadic version of partition today. I settled on the following version: ...
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Writing monadic Haskell to evaluate arithmetic expression

I have been studying Haskell by myself for about a little over a year. And I have been stuck at monad/monad transformers for quite a while until recently some examples I read online enlightened me. So ...
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35 views

Using foldl to add numbers, while validating that they are nonnegative

I have a function that will calculate the sum of a list of integer, however if any of the number is negative, it will should an error instead (not throwing exception). The following is a working ...
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Stack Exchange API Reader

I've been working on some code to pull questions from the realtime feed on stackexchange.com and query more information about them from the API. It works, but I'd love some feedback on how I could ...
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1answer
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Implement State Monad transformer in Haskell from scratch

When I was studying Monad Transformer, I decided to create StateT s m a from scratch with instances for Functor, ...
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1answer
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Determining whether (x,y) points in the input constitute a function

I'm learning Haskell, and I've finally gotten around to coding up some Haskell. This code passed my tests. This code takes a line from standard in which tells it how many cases it's going to have (1 ...
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Monadic implementation of asynchronous tasks (hidden overhead of threads)

So, I've been reading about monads and I wanted to see if I could implement a system for asynchronous computation in a monadic way. I came up with two solutions: The first one spawns a thread for ...
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Monadic list for a more functional approach to C#

I am learning about monadic structures and have created a sort of monad list that implements the map and binding "operators" (or technically methods) in C#. I understand that Linq already exists and ...
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Haskell/Yesod - Butchering use of MaybeT

The following is some code that is used to provide the frontend with certain parameters when rendering a form. The parameters are conditional based around what data is being changed, ...
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F# Either computation expression with while loop

I want to implement an Either computation expression in F#. The computation executes a function inside a while loop, and this function's return type is ...
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608 views

State Monad in TypeScript

Here's my implementation of State Monad in TypeScript, based on a canonical Haskell implementation. I would like it to get code reviewed. ...
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1answer
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Monad Transformers

I've run into the same scenario involving Monad Transformers a few times now, and I'm looking for some "best-practice" advice. Bear with the introduction, the issue itself is really simple: Say you'...
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Simple DFA using the State Monad

I've been getting to grips with some Monads in Haskell recently, and to get some practice I plucked a Mealy Machine out of thin air and decided to implement it using the State Monad (it's a simple ...
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177 views

Bubblesort using Writer monad

I still consider myself a newbie to functional programming, and I still have some trouble wrapping my head around a lot of functional concepts. Anyway, here goes: ...
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Haskell limit number of recursions

I want to create a monad which will prevent more than 10 instances of recursion (which I am implementing by preventing more than 10 instances of >>=). My <...
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Monad Implementation in Typescript

Having been introduced to monads through Scala, I am trying to add the generalized design pattern to my toolkit by implementing it in a language I use more regularly (typescript). My tests pass (...
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Haskell Sudoku Solver

Any comments on the following Sudoku solver? Comments I'm particularly interested are (but not limited to...) Algorithm. It created a list of "potentials" for each cell, and trims them down until it'...
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Sudoku solver in Haskell using Reader+ST

While trying to understand monad transformers in Haskell I came up with a sudoku solver based on a combination of the ST and Reader monad. I am positively surprised with the performance, however ...
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Leap year check in Haskell, using pattern matching or bind operator

One of the first Haskell puzzles on http://exercism.io/ is to implement a leap year check. I've done this twice. Using pattern matching: ...
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Monadic transformation on a list, done in three ways

I am working my way through the Haskell Book and ran into the exercises at the end of the Monad chapter(18). One in particular asked to define a function with the following signature: ...
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1answer
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CompletableFuture Chain (Monad)

I wrote an application that has a modular structure. Each module communicates with different remote servers. Time for execution of a task in each module varies. So communication should be asynchron. ...
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312 views

Algebraic Effect Monad to separate pure and impure code

I want to write my C# code in a way which makes effects (state, I/O, etc) explicit in the type signature. I have started to understand monads, and have some acquaintance with algebraic effects, though ...
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Using monads and monad transformers (simple matching engine)

Like most of the folks here, I'm learning Haskell, so I figured an interesting exercise would be to build a matching engine that I could use to build a financial exchange. I'm wondering primarily if I ...
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584 views

Composing IO and Async in haskell

Whether based on MVar or TVar, async implementation are always based on operation on some underlying monad IO and STM. Making Async a monad on its own, as in F# async computation builder, if done in ...
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419 views

Transforming XML to HTML with XSLT

I'm still learning from designing with types non strings by Scott Wlaschin. For my project I transform some XML to HTML with xslt. Because every filename can have side effects, they are stored in ...
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795 views

Prolog parser written in Haskell

I am writing a very simple Prolog intepreter in Haskell. It's a class assignment and I really want to do it right. I was able to (quite quickly) write a parser for the language. Today I borrowed a ...
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Struct implementation of immutable Maybe<T> monad

I have developed the following implementation of an immutable struct Maybe<T> monad for use in my game project; as an ...
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1answer
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CIS194: Risk!-style "battles"

After reading LYAH and using RWH as a reference, I've been doing the exercises from CIS194, which is often suggested to beginners on irc://freenode.net/#haskell and ...
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Hacky Haskell monadic testing

Edit for posterity: The library you're looking for is Hspec. I recently started writing Haskell tests with HUnit. My general setup looks something like this: ...
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DSL design -- C DSL

I am trying to design a DSL for writing C. Here is my code as a GitHub gist (Github does not allow me to have slashes in the filenames so I have used underscores instead). It depends on packages free ...
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1answer
373 views

Simple/Naive Implementation of Identity and Maybe Monad in Elixir

This is my first crack at a identity monad and a maybe monad in Elixir: ...
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1answer
103 views

Type Validation using State Monad

I am attempting to build a compiler for a stack-based (or concatenative) programming language in Haskell. I've done fine getting the lexer and parser up and running, but now that I'm into the meat of ...
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353 views

Applicative permutation to generate knight moves

I want to understand Haskell better by trying to stretch possibilities of Functor, Applicative, and Monad as much as possible and study how they behave. So in Learn You a Haskell there's an exercise ...
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354 views

Functional Programming Library

I have started putting together a functional programming library in C# which is somewhat inspired by scalaz and the book Functional Programming in Scala. I am hoping to get some feedback on the Monad ...
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107 views

Safe stack operations with lens

I wrote this code to learn how to work with State monad. Then I added Either result to gracefully handle error condition (stack underflow). I believe most of this code is actually unnecessary. I know ...
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1answer
82 views

Polymorphic components for graphics and program state

I asked this question on StackOverflow, got some answers, most notably a link to this one, and basing on that I've implemented this: ...
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Checking in four directions for board game win

I have the following code which I would like to simplify in Haskell, although I'm not sure how. I recall that I can use monads to simplify a case chain when the result of the case leads onto a next ...
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Haskell: downpair :: (Monad m) => m (a, b) -> (m a, m b) implementation

I'm trying to implement function downpairs which takes monad of pairs and transform it into pair of monad My current implementation is ...