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HASKELL QUESTIONS

All do blocks in mapM are executed before main computation inside do block
All do blocks in mapM are executed before main computation inside do block
Any of those help This is lazy evaluation. The value decrBytes is not required within decryptProgress, so it isn't "forced" (i.e. actually evaluated). Internally it is represented as a "thunk"; a data structure containing a function pointer and the n
TAG : haskell
Date : January 12 2021, 01:40 AM , By : Pieter Taelman
Is there a data structure where using bottom is needed to have a correct type?
Is there a data structure where using bottom is needed to have a correct type?
it helps some times Sometimes, you want to pass a value just to pass the type of that value, while you don't care about the actual value. Take for example asTypeOf :: a -> a -> a. It does not examine is seconds argument, it's just there to set the ty
TAG : haskell
Date : January 11 2021, 03:32 PM , By : ziqew
Automagically create predicates that detect components of a sum type?
Automagically create predicates that detect components of a sum type?
hope this fix your issue GHC.Generics is another way.You can use the is function from the lens library and generic prisms from the generic-lens library to check whether a value starts with a given constructor MyCon:
TAG : haskell
Date : January 11 2021, 03:26 PM , By : Lee KW
Typeclass instance not found for generic
Typeclass instance not found for generic
Hope this helps The problem is that GHC is unable to match c with 'MetaSel, because 'MetaSel is of a higher kind. By default fresh type variables are assumed to have kind *, so the matching fails here.One way to fix this is to just replace c with 'Me
TAG : haskell
Date : January 10 2021, 02:06 PM , By : platformNomad
NTLM authentication in Haskell
NTLM authentication in Haskell
This might help you I haven't tried this and don't know NTLM, but it looks like the libcurl bindings have what you need.Maybe you could use them to construct something like this with it. You could try:
TAG : haskell
Date : January 10 2021, 02:04 PM , By : uxn
Why isn't sum == foldl1 (+)?
Why isn't sum == foldl1 (+)?
I wish did fix the issue. Because the Data.Matrix instance for Num implements fromInteger by returning a 1x1 matrix, and + by adding elementwise which truncates the right matrix to the size of the left one (or errors if the left is larger than the ri
TAG : haskell
Date : January 10 2021, 02:04 PM , By : jaredsmiller
Usage of ! in a Haskell type
Usage of ! in a Haskell type
help you fix your problem It´s a strictness flag as oposed to lazy evaluation.
TAG : haskell
Date : January 10 2021, 01:49 PM , By : user96271
backtracking in haskell
backtracking in haskell
To fix the issue you can do Would it help you to use an Array type here, rather than a list-of-lists? You are still doing functional programming, just using a better data structure. You can create an Array (Int,Int) Int if that works for you. See:
TAG : haskell
Date : January 10 2021, 01:47 PM , By : Rik
How can I classify exceptions purely?
How can I classify exceptions purely?
I hope this helps you . I think perhaps your question is more about style than substance, which is why it's not received much attention - the end point is somewhat unclear. Some of your attempts could be tidied, eg.
TAG : haskell
Date : January 06 2021, 03:27 AM , By : FriendL
Is posible to create an infinite wrapper in Haskell with Rank N types?
Is posible to create an infinite wrapper in Haskell with Rank N types?
should help you out Yes and no!First of all, your type is inaccurate in the signature of the function. Taking your example of wrapper 4 'a', the return type of the function is m (m (m (m a))) (where m is []), not m a.
TAG : haskell
Date : January 06 2021, 03:27 AM , By : ffmmjj
Why is a universally quantified variable not in scope in this Haskell function?
Why is a universally quantified variable not in scope in this Haskell function?
I hope this helps you . In standard Haskell, type variables cannot have a scope that extends beyond the type signature in which they appear. Any variables which isn't explicitly given a smaller scope with forall1 is presumed to scope over the type si
TAG : haskell
Date : January 03 2021, 08:18 AM , By : Ashley
Retrieve values from datatype
Retrieve values from datatype
I wish did fix the issue. The only way to get values out of a data constructor is indeed to pattern-match on it. If you want, you can define a function that does it, as you have done with nodeId, or you can just do the pattern matching inline. For ex
TAG : haskell
Date : January 02 2021, 06:48 AM , By : John Q.
How to get better error messages with type-families?
How to get better error messages with type-families?
wish of those help Sure it's possible to provide better compile-time error message using custom type errors. I've described how to use them in my blog post: A story told by Type Errors
TAG : haskell
Date : January 02 2021, 06:48 AM , By : ERaubenheimer
How do I import a package I have installed from stackage?
How do I import a package I have installed from stackage?
wish helps you to Willem Van Onsem for pointing out that I need to be importing modules, not packages. In this case, import Data.Text.Prettyprint.Doc was the appropriate choice.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : CrookedNumber
Difference between -> and => symbols. What do they mean?
Difference between -> and => symbols. What do they mean?
hop of those help? -> is for explicit functions. I.e. when f is something that can be written in an expression of the form f x, the signature must have one of these arrows in it†. Specifically, the type of x (the argument) must appear to the left of
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user187383
Haskell List functions
Haskell List functions
help you fix your problem Haskell's function has two couples of list functions: head :: [a] -> a, and tail :: [a] -> [a]; and init :: [a] -> [a] and last :: [a] -> a.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user133629
doing trivial search and replace via a regex
doing trivial search and replace via a regex
Hope that helps You can use the (=~) form that returns the "before", "matched", and "after" parts. To replace only the first occurrence, use:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Frank Bradley
How do you include a multiline number in a Haskell source file?
How do you include a multiline number in a Haskell source file?
Does that help You could make a quasiquoter. Might even want to upload it to hackage:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user106284
How to accumulate changes without renaming variable?
How to accumulate changes without renaming variable?
like below fixes the issue I have a list of Strings, errors. I do some checks and if any fail, I append a message to errors. Something like this: , You could group the conditions and messages together
TAG : haskell
Date : January 02 2021, 06:48 AM , By : afarouk
Do monad transformers, generally speaking, arise out of adjunctions?
Do monad transformers, generally speaking, arise out of adjunctions?
Hope this helps The three constructions in this answer are also available, in a reproducible form, in this Gist.Simon C's construction...
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Mistere
How to run Haskell on GraalVM?
How to run Haskell on GraalVM?
it helps some times This is not a complete answer, but it's most of the way there. It is not too complicated:The Haskell runtime is just a library that you can find in your GHC installation. On my Mac it's in $PREFIX/lib/ghc-$VERSION/rts, where $PREF
TAG : haskell
Date : January 02 2021, 06:48 AM , By : jaset
"cabal new-install Cabal cabal-install" answered "cabal.exe: Could not read index. Did you call 'checkFor
"cabal new-install Cabal cabal-install" answered "cabal.exe: Could not read index. Did you call 'checkFor
will help you I have tried the "cabal -v3 v2-update" and it showed me the file that was missing and its path. At first I tried to restore the file by hand, but someone suggested erasing all the files related to the missing one, and, voilá, cabal rege
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Stephen
How to write this code shorter and so that it may be used for a wider purpose?
How to write this code shorter and so that it may be used for a wider purpose?
hop of those help? It does not make much sense to use a list in the first place. A list should be used for a colleciton of items. But here "Mathematics" (the Name), and "BSc" (the Type) are orthogonal items. They only "happen" to have the same type.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : SpittingCAML
Why provide the `HasCallStack` mechanism since we already have `ghc -prof -fprof-auto-top` in GHC?
Why provide the `HasCallStack` mechanism since we already have `ghc -prof -fprof-auto-top` in GHC?
will help you AFAIK, there are two ways to get a call stack for debugging in Haskell: , HasCallStack has a few basic advantages:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Martin
"Generalized arrows" and proc notation?
"Generalized arrows" and proc notation?
should help you out I've wondered about that before, too. But – proc notation is so widely considered a silly oddball that there's probably not much interest in generalisation either (though I daresay this is what would make it actually useful!)Howev
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Kilimanjaro
How values are parsed to this insert function in haskell
How values are parsed to this insert function in haskell
With these it helps This represents a set as a function that, given an Int, tells you if it is in the et (by returning True) or not (by returning False).The empty set is thus the function that returns False no matter what argument you give.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Magnus
Is there a standard Haskell function with type: (Floating a, RealFrac b) => a -> b?
Is there a standard Haskell function with type: (Floating a, RealFrac b) => a -> b?
wish of those help I need to call floor() on a value, which is only constrained to be of class Floating, but floor() requires RealFrac. , Consider the following instance of Floating:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : glisignoli
Is it possible to design Stack (Haskell) just like NPM (NodeJS)?
Is it possible to design Stack (Haskell) just like NPM (NodeJS)?
This might help you Yes, it is possible. In fact, cabal used to work that way by default. It was abandoned in the move to nix-style builds, but as far as I know there's no technical problem standing in the way of doing it again.That said, my experien
TAG : haskell
Date : January 02 2021, 06:48 AM , By : fstender
Haskell: Order of evaluation for (++) and (:) operators
Haskell: Order of evaluation for (++) and (:) operators
I think the issue was by ths following , Both the (++) :: [a] -> [a] -> [a] and (:) :: a -> [a] -> [a] function have 5 as precedence, as we can read in the Haskell '10 report, and are right associative.We can also obtain this data with :i in the ghci
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Bimal Poudel
Is there anything wrong with wrapping Data.Sequence in my own Queue type?
Is there anything wrong with wrapping Data.Sequence in my own Queue type?
To fix this issue Nothing wrong at all - in fact, that's precisely what newtype is for!Ideally, your types should reflect your intentions - whoever reads your code (including yourself, in the future, when you have forgotten what you did and why) shou
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Mare Astra
Is Traversable different for breadth-first and depth-first trees?
Is Traversable different for breadth-first and depth-first trees?
may help you . Traversable has to agree with Foldable. Specifically, if Monoid m, then Applicative (Const m), causing a consistency law foldMap f = getConst . traverse (Const . f). It is thus impossible for Breadth and Depth to share a Traversable. T
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Atanas
Why doesn't Gloss render in native resolution?
Why doesn't Gloss render in native resolution?
it should still fix some issue Gloss can be used with native resolution under OSX with hdpi-display when the default window management backend GLUT is exchanged for GLFW. To do this rebuild Gloss with the appropriate flags:cabal install -f -GLUT -f G
TAG : haskell
Date : January 02 2021, 06:48 AM , By : dnyaneshwar
Quantified constraints vs. (closed) type families
Quantified constraints vs. (closed) type families
seems to work fine I don't think you can do such thing, but I could certainly be wrong. In your example you are missing an extra constraint Show (f a) in order for it to be complete:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : kbrust
Increment positive numbers and decrement negative number all by 1
Increment positive numbers and decrement negative number all by 1
fixed the issue. Will look into that further $ is going to bind looser than any other operator, so you need explicit parentheses. Also, you don't need to deconstruct the list since you're just putting it back together (the x:xs pattern is unnecessary
TAG : haskell
Date : January 02 2021, 06:48 AM , By : techthumb
Example of non-trivial functors
Example of non-trivial functors
wish of those help Here's kind of a fun argument I just stumbled on. (It's late so I wonder if it will be sensical tomorrow)We can construct the type of proofs of SK reducibility as a GADT:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user185751
Defining append using foldr
Defining append using foldr
To fix this issue foldr needs three arguments: an accumulator function, an initial accumulator value, and a list (well, a Foldable).foldr (:), therefore, requires two more arguments: initial accumulator, and list.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user183345
haskell : calculated fields in custom data types
haskell : calculated fields in custom data types
it helps some times Is it possible to do something that is equivalent to having a field in a data type that is automatically calculated from other fields in the data type. For example: , Without any additional requirement, that's simply a function.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Nathan Good
making permutation from two list but not using full number
making permutation from two list but not using full number
it helps some times Making a guess at what your current code does, here's a proposal for an algorithm: nondeterministically pick a permutation of the vowels assign indices to the permutation use the standard tails trick to nondeterministically choose
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Marcos de Carvalho
Using "Either" in Data Validation
Using "Either" in Data Validation
wish helps you Either's Monad and Applicative instances can't accumulate errors: by law, they must stop at the first Left value. So if you want to use Either to accumulate errors, you have to do it by hand, not through the Applicative or Monad instan
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Hunter
screen doesn't show, Console.ReadLine() in Haskell?
screen doesn't show, Console.ReadLine() in Haskell?
This might help you getLine reads a whole line (returning that as a string, but you don't need that).
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Jason Jennings
Inferred generic function typechecks as a return type but not an argument type
Inferred generic function typechecks as a return type but not an argument type
this one helps. I'm learning about SYB and rank n types, and came across a confusing case of what seems like the monomorphism restriction. , In your second snippet, op is actually not polymorphic.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Chaz
How to make type conversion in Haskell?
How to make type conversion in Haskell?
wish help you to fix your issue In your call truncate (sqrt 5) + mod 5 2, the 5s have different types. Indeed the 5 in the sqrt 5 should have as type Floating a => a, whereas the 5 in mod 5 2 has type Integral b => b. Although it is, strictly speakin
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user186831
How can I read mutiple lines from stdin within GHCI more than once?
How can I read mutiple lines from stdin within GHCI more than once?
around this issue In contrast to the information in "learn you a haskell", on my windows system, ghci translates CTRL-D to EOT, not EOF. , Here is the function I was looking for:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Brazen
Custom Filter Function with Predicate using List Comprehension
Custom Filter Function with Predicate using List Comprehension
This might help you I need to develop my own filter function similar to how filter works in Haskell but by using list comprehension and a predicate. So I would put lcFilter (>3) [1,2,3,4,5,6,10,444,3] in ghci and it would print all numbers greater th
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Munir
Listing Items in Haskell
Listing Items in Haskell
I hope this helps you . Assuming Item.Item is the name of a data constructor with record syntax, you simply need to use it twice.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : SachinJadhav
Insertion in a binary search tree(data stored at leaf node only)
Insertion in a binary search tree(data stored at leaf node only)
should help you out The technique you require to solve this problem is called pattern matching combined with conditional branching (if / pattern guards).You can do this either by using a case statement or by defining alternative function definitions
TAG : haskell
Date : January 02 2021, 06:48 AM , By : woxorz
How can I concisely match a list according to its latter half, and bind it's former half accordingly?
How can I concisely match a list according to its latter half, and bind it's former half accordingly?
I hope this helps you . On built-in String, this is a very bad idea, since your pattern match turns out to be quite expensive. On other string types, like Text or ByteString, you can use pattern guards:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : pcoramc
How to display hashmap in Haskell
How to display hashmap in Haskell
To fix the issue you can do Given that the keys k and the values v are instances of Show, then a HashMap is an instance of Show as well. Indeed in the source code, we see:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Marcos de Carvalho
Replace element in list
Replace element in list
hope this fix your issue you don't do mutation on the list but create a new list from the given list.an example, without any error handling...
TAG : haskell
Date : January 02 2021, 06:48 AM , By : al.
Good examples of not a Contravariant/Contravariant/Divisible/Decidable?
Good examples of not a Contravariant/Contravariant/Divisible/Decidable?
should help you out The Contravariant family of typeclasses represents standard and fundamental abstractions in the Haskell ecosystem: , (partial answer)Not contravariant
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Sebastián Ucedo
Catamorphism that allows looking at part of the final result
Catamorphism that allows looking at part of the final result
Hope this helps Though this is not necessarily what you were looking for, we can encode the laziness trick with a hylomorphism:
TAG : haskell
Date : January 02 2021, 06:48 AM , By : goffi
Implementing monad on recursive type
Implementing monad on recursive type
it should still fix some issue To overcome your specific challenge, you should try pattern matching on the result of f x.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Meg
Why doesn't Haskell allow more complex infix expressions?
Why doesn't Haskell allow more complex infix expressions?
this one helps. Why doesn't Haskell allow more complex infix expressions, eg. , It can be faked
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Waheedoo
(new to haskell) if statment generating parse error
(new to haskell) if statment generating parse error
I wish did fix the issue. In Haskell, if clauses are expressions. That implies that they need to always evaluate to a value, and also to have a definite type. More specifically, if clauses without an else are syntax errors. Also, the then and else ca
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Jorge Palacio
Default type of id function
Default type of id function
should help you out In short: id [] has type [a], the exact type will depend on the context of the caller.We can perform the reasoning of the types ourselves. We basically have two items here: id and [], id has as type id :: a -> a, and [] is just a
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user116330
How to get element's index in list when using foldl
How to get element's index in list when using foldl
this will help We can implement this in a more simple way. We can consider a infinite list where we repeat two functions: id :: a -> a, and negate :: Num a => a -> a, and use cycle :: [a] -> [a] to construct an infinite list. So cycle [id, negate] wi
TAG : haskell
Date : January 02 2021, 06:48 AM , By : General Mills
The relationship between generic-lens and DataKinds GHC extension
The relationship between generic-lens and DataKinds GHC extension
it fixes the issue DataKinds is an extension that allows data types to be promoted to the type-level. What this means precisely is kind of a can of worms (it's one step on the way to dependent types), so I'll try and keep this explanation focused on
TAG : haskell
Date : January 02 2021, 06:48 AM , By : user176691
Define a function replic lt in Haskell
Define a function replic lt in Haskell
This might help you What you could do is solve the problem using explicit recursion and then try to detect places where you can replace certain parts of the recursion with a higher-order function. Here are some hints for creating an explicitly recurs
TAG : haskell
Date : January 02 2021, 06:48 AM , By : eferro
Haskell Bool to Char function seems to have the wrong type
Haskell Bool to Char function seems to have the wrong type
wish help you to fix your issue Variable names must start with lower-case letters (or non-: symbols if they are infix names), so that the compiler can easily distinguish them from constructors.
TAG : haskell
Date : January 02 2021, 06:48 AM , By : Josh Tegart
Do IO operations run in green threads?
Do IO operations run in green threads?
will help you From the documentation of Control.Concurrent.Async
TAG : haskell
Date : January 02 2021, 06:48 AM , By : alexmajy

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