December 2017 - Beginners

IO question
by mike h 11 Dec '17

11 Dec '17

I have input <- readFile “data.txt” let input’ = splitOn “,” input …. How do I make that into just one line? Thanks Mike

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What does 'impossible happened' mean?
by Baa 11 Dec '17

11 Dec '17

Hello, List. I hit error: ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): initTc: unsolved constraints WC {wc_insol = [W] spec_a86e :: t_a86d[tau:1] (CHoleCan: spec)} Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug what does it mean and how to fix it? PS. I only added template Haskell "call" in my test suite like $(blahBlah something) The same call works fine in other modules and in GHCI. "stack build" works too, but tests - no (last stack 1.6.1) === Best regards, Paul

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How to call constructor from Template Haskell
by Baa 11 Dec '17

11 Dec '17

Hello All! I have function which constructs some data type. It has signature `Name -> Q [Dec]`. Somewhere in its body I'm extracting constructors of some type with pattern-matching: case tyCons of DataD ctx nm tyVars mbKind cs derivs -> ... Type of those constructors `cs` instantiates some class like this: class MyClass a where specialValue :: a So, I'm iterating over those `cs` but I want to skip one of them which is equal to `specialValue`. Something like this: [c | c <- cs, c /= specialValue] How to do this with Template Haskell's `Con` type (`c`::Con) - I can't call it to compare created value with a `specialValue` ? === Best regards, Paul

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Lost Monad Signature
by Quentin Liu 09 Dec '17

09 Dec '17

Hi, The function `join` flattens a double-layered monad into one layer and its type signature is join :: (Monad m) => m (m a) -> m a But when the first argument supplied is `(,)`, the type signature becomes join (,) :: b -> (b, b) in ghci. The monad constraint is lost when supplied the first argument. So my question is why the type constraint is lost and what monad is supplied here. Regards, Qingbo Liu

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Monoid algebra
by Manny Romero 09 Dec '17

09 Dec '17

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Template Haskell: simple way to "declare" type and instance
by Baa 08 Dec '17

08 Dec '17

Hello, All! Is any way similar to templates (like in Crystal language, Rust, etc) to create `data X = ...`, `instance Show ...`, etc in TH? I found this tutorial https://wiki.haskell.org/A_practical_Template_Haskell_Tutorial but all looks complicated. Or something like Lisp macros, may be some modern library? Like quasy-quotations but for `data` and `instance` declarations (I don't know is it possible even) ? === Best regards, Paul

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Stack could not find libHStransformers
by Quentin Liu 07 Dec '17

07 Dec '17

Hi, I was trying to use Parsec and imported Text.ParserCombinators.Parsec. I could call “stack ghc” to compile the file, but when I loaded the file into ghci and called any function, ghci would report errors > can't load .so/.DLL for: /Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib (dlopen(/Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib, 5): Library not loaded: /usr/local/opt/ghc/lib/ghc-8.0.2/transformers-0.5.2.0/libHStransformers-0.5.2.0-ghc8.0.2.dylib > Referenced from: /Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib > Reason: image not found) > I tried to install transformers by calling `stack install transformers` but the problem still persisted. Is it because the version of transformers in my repo is different from the one referenced by `mtl` package? How should I fix it? Regards, Qingbo Liu

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Little question about "forall"
by Baa 07 Dec '17

07 Dec '17

Hello everyone! Suppose I have: class C a where someA :: a f :: C a => a -> Int f a = let x = someA :: a in -- BUG!! 0 BUG as I understand I due to `:: a` - this is another `a`, not the same as in `f` singature. But seems that it's the same `a` if `f` is the "method" of some instance. And such bug does not happen if I return it, so my question is how to create such `x` of type `a` in the body of the `f`? How to use `a` anywhere in the body of `f`? Is it possible? I found a way if I change signature to `forall a. C a => a -> Int`. But there are another questions in the case: - as I know all such params are under "forall" in Haskell by-default. Seems it's not true? - in method body seems they are under "forall" but in simple functions - not? - i'm not sure what exactly does this "forall", IMHO it unbounds early bound type's variables (parameters) by typechecker, but how `a` can be bound in the just begining of signature (where it occurs first time) ?! As for me, looks that w/o "forall" all `a`, `b`, `c`, etc in the body of simple functions always are different types. But w/ "forall" Haskell typechecker makes something like (on type-level): let a = ANY-TYPE in ... here `a` can occur and will be bound to ANY-TYPE Where am I right and where am I wrong? :) === Best regards, Paul

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Re: [Haskell-beginners] Unpacking tuples
by Frerich Raabe 04 Dec '17

04 Dec '17

On 2017-12-04 20:05, mrx wrote: > Hi, > > If I have a function that produce tuples with three members and I want those > members as parameters for a function that takes three parameters. > How would I unpack that tuple? > > It seems that curry does the trick for tuples with two members. > > How do I do this when there are more than two members? There are functions like 'curry3' ( http://hackage.haskell.org/package/utility-ht-0.0.14/docs/Data-Tuple-HT.htm… ) but I think if you don't need this very often, it might be easiest to just go for let (a,b,c) = f x in g a b c -- Frerich Raabe - raabe(a)froglogic.com www.froglogic.com - Multi-Platform GUI Testing

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Unpacking tuples
by mrx 04 Dec '17

04 Dec '17

Hi, If I have a function that produce tuples with three members and I want those members as parameters for a function that takes three parameters. How would I unpack that tuple? It seems that curry does the trick for tuples with two members. How do I do this when there are more than two members? Patrik

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