Recent questions tagged haskell

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haskell - Understanding this definition of HList

I'm relatively new to Haskell, and I'm trying to understand one of the definitions of HList. data instance HList '[] = HNil newtype instance HList ... with two fields) besides avoiding boxing of HCons1? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What is the significance of algebraic datatypes with zero constructors?

This passage, which unfortunately lacks references, about the development of ADTs in Haskell, from A History of Haskell: Being Lazy With Class, section 5.1: ... wondering, how would such an ADT be useful? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Prolog implementation of Quine's algorithm for classical propositional logic (in Quine's "Methods of Logic")

I know only one prover that translates the algorithm that Quine gave for classical propositional logic in his book Methods of Logic (Harvard University ... published but not reproduced in this page. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Could not deduce KnownNat in two existentials with respect to the singletons library

I was experimenting with the singletons library and I found a case that I don't understand. {-# LANGUAGE GADTs, StandaloneDeriving, RankNTypes, ScopedTypeVariables, ... a1 (GHC.TypeLits.CmpNat a a1)))) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why wrapping the Data.Binary.Put monad creates a memory leak?

I'm trying to wrap the Data.Binary.Put monad into another so that later I can ask it questions like "how many bytes it's going to write" or "what ... : There is also a second part to this question here. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to show utf8 text with snap and heist?

I have used writeBS writeText from Snap and renderTemplate from heist but none of them seems to support unicode. site :: Snap () site = do ifTop (writeBS ... in fact its output is just some messy code. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - MacOSX - cabal install: built failed during the building phase. The exception was: ExitFailure 1 - dist/package.conf.inplace: inappropriate type

using cabal; came accross this error. did cabal install and this happened. failed during the building phase. The exception was: ExitFailure 1 checked ... Glasgow Haskell Compilation version: 7.8.4 See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Gtk2hs windows 7 installation

I did: cabal update cabal install gtk2hs-buildtools But stuck here: >cabal install gtk Resolving dependencies... [1 of 2] Compiling SetupWrapper ( C: ... ghc-pkg uregister. Now see th updated answer. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why do we use tuples, if we can use a two dimensional list?

Is there a benefit to specifically reserving varied data-type pairings for tuples like such: [(23, "Jordan"), (8, "Bryant")] As opposed to just using ... the second piece of code will not work in Haskell See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Issue installing Snap on macOS Sierra

I'm having trouble installing the Haskell framework Snap on macOS Sierra. Whenever I install it, I get the following message: Resolving dependencies... ... using sudo and deleting my ~/.ghc file See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - M.Map sudden expected type error

Everything was working great up until about a month or so ago... Suddenly I'm getting berkson.github.io/source/blog.hs: 333, 42 ? Couldn't ... com/berkson/berkson.github.io/blob/source/source/blog.hs#L330 See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How do I get Parsec to let me call `read` :: Int?

I've got the following, which type-checks: p_int = liftA read (many (char ' ') *> many1 digit <* many (char ' ')) Now, as the function name ... pairs in proteins. I have no control over the file format!) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why are these family instance declarations conflicting?

I'm getting an error for this code, and I don't understand where the conflict is. {-# LANGUAGE TypeFamilies, FlexibleContexts, FlexibleInstances, ... hs:29:8 Failed, modules loaded: none. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Getting the data constructor name as a string using GHC.Generics

I'd like something like the following: constrName :: Data a=> a -> String constrName = showConstr . toConstr But for GHC.Generics. I see the Constructor class, but ... in scope. I'm using base-4.8.1.0. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why wrapping the Data.Binary.Put monad creates a memory leak? (Part 2)

As in my previous question, I'm trying to wrap the Data.Binary.Put monad into another monad so that later I can ask it questions like "how many ... infinite loop :-|. Could someone please help further? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Lazy decoding of a list with Data.Binary

I am lazily encoding lists using this code (taken from this SO question): import Data.Binary newtype Stream a = Stream { unstream :: [a] } instance Binary a ... this not lazy, and how I can make it lazy? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How does the <<loop>> error "work" in detail?

I'm working on this tool where the user can define-and-include in [config files | content text-files | etc] their own "templates" (like mustache etc) and ... results for "how to loop in Haskell" etc..) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - looking for a function like break that include the last element

Something that should be easy but somehow I don't find how to do it... let's have the following string: "0@workspace_command-7a86cb8b-0987-fe40-61f2- ... ","-7a86cb8b-0987-fe40-61f2-42b0f6fc3e1c"]... See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to get ReaderT to work with another monad transformer?

I would like to embed ReaderT into another monad transformer. How do I do this? The example below uses Scotty but I think it would be the same with ... Actual type: ScottyT Text (ReaderT Config IO) () See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Type of a double

Learning Haskell, in ghci: Prelude Data.Ratio> :type 0.15 0.15 :: Fractional a => a Prelude Data.Ratio> 0.15 0.15 it :: Double Why are ... ? Are those two instances of 0.15 actually different types? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to fix "Illegal datatype context" (use -XDatatypeContexts)?

I am a new learner of Haskell, my code is as follows: data Num a=>Units a = Units a (SymbolicManip a ) deriving (Eq) I am not sure how to fix it? Anyone can help me? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell sum type multiple declaration error

data A=A data B=B data AB=A|B Which makes a sum type AB from A and B. but the last line induces a compile error "multiple declarations of ... ghc disallows me from making sum types for user-defined types? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to build matrix of zeros using hmatrix?

Trying to use hmatrix, to create a zero marix. For some reason, when I try this on command line, it works: buildMatrix 2 3 ((r,c) -> fromIntegral 0) However ... -> ...); m } Failed, modules loaded: none. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Get value from IO rather than the computation itself

Being quite new to Haskell, I'm currently trying to improve my skills by writing an interpreter for a simple imperative toy language. One of the expressions in this ... with IO I see no way out of that..? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Type error when testing a function with a negative number

I am following along with the Learn you a Haskell for great good, I have implemented take': take' :: (Ord i, Num i) => i -> [a] -> [a] take' n _ | n <= 0 = [] ... [a] -> [a]), Ord i) => i -> [a] -> [a] See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Cabal install criterion out of memory

I'm running on a container with 768MB ram and 512 MB swap space. I can't increase either of this. cabal install criterion always gives Failed during the ... this or do I have to make do without criterion? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Take elements on even positions from the list

Problem: using fold, take from the list elements which are on the even positions: GHCi> evenOnly [1..10] [2,4,6,8,10] GHCi> evenOnly ['a'.. ... idea is correct, I just did something wrong with the syntax. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What approach to error handling to use with pipes(-core)?

I'm currently writing some pipes-core/attoparsec plumbing for a small project of mine. I want each parser to give a pipe that awaits ByteString input ... or even be equivalent to the fourth one. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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