monads | 👻 Option , Result , and Either types | Functional Programming library
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Option, Result, and Either types for JavaScript.
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QUESTION
I more or less wrapped my head around monads, but i can't deduct how expression
...ANSWER
Answered 2021-Jun-12 at 06:43This follows from how >>= is defined for the ((->) r) types:
QUESTION
I'm stuck on a problem with writing a parser in Haskell that I hope someone can help out with!
It is a bit more complicated than my usual parser because there are two layers of parsing. First a language definition is parsed into an AST, then that AST is transformed into another parser that parses the actual language.
I have made pretty good progress so far but I'm stuck on implementing recursion in the language definition. As the language definition is transformed from AST into a parser in a recursive function, I can't work out how it can call itself if it doesn't exist yet.
I'm finding it a bit hard to explain my problem, so maybe an example will help.
The language definition might define that a language consists of three keywords in sequence and then optional recursion in brackets.
...ANSWER
Answered 2021-Jun-10 at 18:53I believe you can use laziness here. Pass the final parser as a parameter to transformSyntaxExprToParser, and when you see a Recurse, return that parser.
QUESTION
I am a beginner in Haskell and I am still studying and I stopped at the problem that has to do with monads.
The problem is that I want to show the comment writer in the template.
I can't do that because when I add a comment from the database it is always in the Handler monad.
Maybe my whole idea is wrong I don't know.
This is the comment entity:
...ANSWER
Answered 2021-Jun-05 at 15:13you should be able to use it in the same way you use comments there: bind it to a local value and access this in your hamlet-file/code.
Of course you want to pair this up with the comment itself so I'd propose something like this:
QUESTION
I am writing a DSL using case classes with the help of the cats.free.Free Monad library. The DSL is to be interpreted by Actors receiving the message, so each actor has to first unwrap a command using Free.resume.
This worked out nicely six years ago using scalaz where we also used the resume function too, but the Functor for the free monad was easy to create as we used case classes that came with an extra function argument that could be mapped over such as k below.
ANSWER
Answered 2021-May-24 at 20:07The problem is in your data type: Get[T] does not use T, so cannot map this meaningfully, other than a no-op: g: Get => Get[B](g)
QUESTION
I know that I can run monadic instructions sequentially inside monads in Kind language, like this:
...ANSWER
Answered 2021-May-24 at 20:48Monads are usually represented by only two operators :
QUESTION
It seems that something has changed in Haskell since the publication of Learn you a Haskell for Great Good. When creating Monads you not only have to define the Functor but also now the Applicative implementation.
In Chapter 14 of the book, in the section on "Making Monads", we define a newtype Prob like...
ANSWER
Answered 2021-May-23 at 22:20If you have a Monad instance for m, you get an Applicative instance of m for free defined by
QUESTION
I am trying to play with MonadState monad and especially with 'modify' function.
When I try
...ANSWER
Answered 2021-May-19 at 19:36As the error indicates, you need to enable the FlexibleContexts extension in Haskell to run this. You can do this by adding a language pragma at the top of your file:
QUESTION
I know I can use state passing and state monads for purely functional mutation, but afaik that's not in-place and I want the performance benefits of doing it in-place.
An example would be great, e.g. adding 1 to a number, preferably in Idris but Scala will also be good
p.s. is there a tag for mutation? can't see one
...ANSWER
Answered 2021-May-17 at 17:15No, this is not possible in Scala.
It is however possible to achieve the performance benefits of in-place mutation in a purely functional language. For instance, let's take a function that updates an array in a purely functional way:
QUESTION
I'm going through The Monad Challenges.
At the section A Missed Generalization, I'm supposed to have at least this code (I've removed parts not relevant to the question), where Gen looks a lot like the State Monad,
ANSWER
Answered 2021-May-15 at 11:10Your solution is probably close to the intended solution, although you might be able to make it more readable by eta-expanding it. You might even consider writing it using do notation, but still use genTwo and mkGen.
As far as I can tell, mkGen is a 'disguised' return function, and genTwo likewise is a 'disguised' monadic bind (i.e. >>=).
The type of generalB (and generalB2) is equivalent to liftM2, which is implemented like this:
QUESTION
I'm new to haskell and functional programming in general and I have a problem with monads. Let's say I have got a list of filenames:
...ANSWER
Answered 2021-May-14 at 13:19Let's start with the simpler list
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