- Beginners - Haskell.org

Re: [Haskell-beginners] Int and Integer
by Peter Normington 17 Dec '17

17 Dec '17

Thanks, Theodore. As simple as that! Best regards, Peter On 17 December 2017 at 15:40, Theodore Lief Gannon <tanuki(a)gmail.com> wrote: > Because: > (!!) :: [a] -> Int -> a > > On Dec 17, 2017 5:50 AM, "Peter Normington" <peter(a)ireby.org.uk> wrote: > >> Why do I get an error in the following example: >> >> memoized_fib :: Integer -> Integer >> memoized_fib = (map fib [0 ..] !!) >> where fib 1 = 1 >> fib 2 = 1 >> fib n = memoized_fib (n-2) + memoized_fib (n-1) >> >> but not in: >> >> memoized_fib :: Int -> Integer >> memoized_fib = (map fib [0 ..] !!) >> where fib 1 = 1 >> fib 2 = 1 >> fib n = memoized_fib (n-2) + memoized_fib (n-1) >> ? >> >> The error is reported by GHCi as: >> >> error: >> • Couldn't match type ‘Integer’ with ‘Int’ >> Expected type: Integer -> Integer >> Actual type: Int -> Integer >> • In the expression: (map fib [0 .. ] !!) >> In an equation for ‘memoized_fib’: >> memoized_fib >> = (map fib [0 .. ] !!) >> where >> fib 1 = 1 >> fib 2 = 1 >> fib n = memoized_fib (n - 2) + memoized_fib (n - 1) >> | >> 2 | memoized_fib = (map fib [0 ..] !!) >> | ^^^^^^^^^^^^^^^^^ >> Failed, 0 modules loaded. >> >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >>

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Int and Integer
by Peter Normington 17 Dec '17

17 Dec '17

Why do I get an error in the following example: memoized_fib :: Integer -> Integer memoized_fib = (map fib [0 ..] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) but not in: memoized_fib :: Int -> Integer memoized_fib = (map fib [0 ..] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) ? The error is reported by GHCi as: error: • Couldn't match type ‘Integer’ with ‘Int’ Expected type: Integer -> Integer Actual type: Int -> Integer • In the expression: (map fib [0 .. ] !!) In an equation for ‘memoized_fib’: memoized_fib = (map fib [0 .. ] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n - 2) + memoized_fib (n - 1) | 2 | memoized_fib = (map fib [0 ..] !!) | ^^^^^^^^^^^^^^^^^ Failed, 0 modules loaded.

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Coredump functionality for Haskell
by Baa 15 Dec '17

15 Dec '17

Hello, All! In C if I need I can abort program and generate coredump - to research state of stack/vars. Is it possible in Haskell - to abort and to generate some dump which can be easy explored/researched in debugger with Haskell symbolic info (not standard Linux coredump)? And to generate it without any coding/hacking? === Best regards, Paul

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Why exception is not catching?!
by Baa 15 Dec '17

15 Dec '17

Hello, dear List! Super simple function - "safe enum conversion". It's based on `Either` because only it instantiates `MonadCatch` (Maybe does not, but with MaybeT I've got error that internal monad Indetity is not MonadCatch, so I avoid to use Maybe). That it is: data A = A1|A2 deriving (Enum, Show) data B = B1|B2|B3 deriving (Enum, Show) conv :: B -> Either SomeException A conv b = safeConv where catchError (e::SomeException) = Left e safeConv = (Right $ toEnum $ fromEnum b) `catch` catchError -- toRes (Left _) = Nothing -- toRes (Right r) = Just r But exception is not catching and I'm getting it in GHCI repl :) Exceptions are different?! What's wrong with this code? === Best regards, Paul

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How to install new GHC ?
by Baa 13 Dec '17

13 Dec '17

Hello, All, I want to switch to the new GHC (8.2.2) because I hit some bug in GHC and suppose that it may be fixed in the new version. So, to try it I changed "resolver: XXX" in my .yaml-files to "lts-9.18". Also I done something else (I don't remember what exactly) and call `stack setup ... --reinstall ...`. New GHC have been installed. And I switched to new LTS - I see it 100%. But when I run `stack build`, IMHO compiller is old, because if I call `stack ghci` I see that old GHC (8.0.2) is calling. I'm not familiar with stack/Haskell tools in-depth and may miss something! For example, I know that LTS-9.18 is last stable, GHC 8.2.2 is last too, but in Stackage seems that 9.18 corresponds to 8.2.2. If I run `stack setup` in the directory with my project again I get: --cut-- stack will use a sandboxed GHC it installed For more information on paths, see 'stack path' and 'stack exec env' To use this GHC and packages outside of a project, consider using: stack ghc, stack ghci, stack runghc, or stack exec --cut-- Would somebody explain me what is wrong here and how I can test a project with new 8.2.2 GHC? === Best regards, Paul

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Re: [Haskell-beginners] IO question
by Vale Cofer-Shabica 12 Dec '17

12 Dec '17

Just to add to David's answer, when I find myself in situations like: foo >>= return . bar hlint helpfully suggests: fmap bar foo So you could also have: fmap (splitOn ",") $ readFile "data.txt" -vale -- vale cofer-shabica 401.267.8253 On Mon, Dec 11, 2017 at 4:24 PM, mike h <mike_k_houghton(a)yahoo.co.uk> wrote: > Thank you David. Before posting I tried readFile "data.txt" >>= … but got > errors as I didn’t use return.!! > > Mike > > > > On 11 Dec 2017, at 21:00, David McBride <toad3k(a)gmail.com> wrote: > > splitOn "," <$> readFile "data.txt" > > or perhaps > > readFile "data.txt" >>= return . splitOn "," > > On Mon, Dec 11, 2017 at 3:56 PM, mike h <mike_k_houghton(a)yahoo.co.uk> > wrote: > >> I have >> >> input <- readFile “data.txt” >> let input’ = splitOn “,” input >> …. >> >> How do I make that into just one line? >> >> Thanks >> >> Mike >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >

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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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