[Haskell-beginners] More type errors I'm having trouble with
Dan Stromberg
strombrg at gmail.com
Thu Dec 3 21:49:48 UTC 2015
Thank you!
You're correct; I had been thinking of do notation as a "multistatement"
thing rather than as a monad thing.
On Wed, Dec 2, 2015 at 5:05 AM, David McBride <toad3k at gmail.com> wrote:
> You seem to be having some problems understanding how Monads and do
> notation work.
>
> do_prefix_hash :: String -> (IO String, String)
> do_prefix_hash filename = do
> hash <- Md5s.prefix_md5 filename :: (IO String)
> (hash, filename)
>
> The above is ill typed. When you open with a do, from that point on the
> type will be Monad m => String -> m Something. But what you intended to
> return is a tuple, which is not an instance of monad. Don't use do in this
> case, just return a tuple.
>
> do_prefix_hash :: String -> (IO String, String)
> do_prefix_hash filename = (Md5s.prefix_md5 filename, filename)
>
> Just look closely at what the error is telling you. Is it expecting a
> type that you told it it returns but it is detecting that your code would
> return something else.
>
>
> On Tue, Dec 1, 2015 at 7:12 PM, Dan Stromberg <strombrg at gmail.com> wrote:
>
>>
>> I'm continuing my now-and-then exploration of Haskell.
>>
>> I'm getting a new crop of type errors that I'm pulling my hair out over.
>>
>> The errors I'm getting are:
>>
>> $ make
>> below cmd output started 2015 Tue Dec 01 04:05:17 PM PST
>> # --make will go out and find what to build
>> ghc -Wall --make -o dph dph.hs Split0.hs
>> [1 of 3] Compiling Split0 ( Split0.hs, Split0.o )
>> [2 of 3] Compiling Md5s ( Md5s.hs, Md5s.o )
>> [3 of 3] Compiling Main ( dph.hs, dph.o )
>>
>> dph.hs:13:13:
>> Couldn't match type `IO' with `(,) (IO String)'
>> Expected type: (IO String, String)
>> Actual type: IO String
>> In a stmt of a 'do' block: hash <- prefix_md5 filename :: IO String
>> In the expression:
>> do { hash <- prefix_md5 filename :: IO String;
>> (hash, filename) }
>> In an equation for `do_prefix_hash':
>> do_prefix_hash filename
>> = do { hash <- prefix_md5 filename :: IO String;
>> (hash, filename) }
>>
>> dph.hs:14:6:
>> Couldn't match type `[Char]' with `IO String'
>> Expected type: IO String
>> Actual type: String
>> In the expression: hash
>> In a stmt of a 'do' block: (hash, filename)
>> In the expression:
>> do { hash <- prefix_md5 filename :: IO String;
>> (hash, filename) }
>>
>> dph.hs:24:23:
>> Couldn't match type `[]' with `IO'
>> Expected type: IO (IO String, String)
>> Actual type: [(IO String, String)]
>> In a stmt of a 'do' block:
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)]
>> In the expression:
>> do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)];
>> .... }
>> In an equation for `main':
>> main
>> = do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> .... }
>>
>> dph.hs:25:20:
>> Couldn't match type `[a0]' with `(String, String)'
>> Expected type: [(String, String)]
>> Actual type: [[a0]]
>> In the return type of a call of `sequence'
>> In a stmt of a 'do' block:
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)]
>> In the expression:
>> do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)];
>> .... }
>>
>> dph.hs:25:20:
>> Couldn't match type `[]' with `IO'
>> Expected type: IO (String, String)
>> Actual type: [(String, String)]
>> In a stmt of a 'do' block:
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)]
>> In the expression:
>> do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)];
>> .... }
>> In an equation for `main':
>> main
>> = do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> .... }
>>
>> dph.hs:25:29:
>> Couldn't match expected type `[[a0]]'
>> with actual type `(IO String, String)'
>> In the first argument of `sequence', namely `io_hash_tuples'
>> In a stmt of a 'do' block:
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)]
>> In the expression:
>> do { buffer <- (hGetContents stdin) :: IO String;
>> let filenames = ...;
>> io_hash_tuples <- map do_prefix_hash filenames ::
>> [(IO String, String)];
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)];
>> .... }
>>
>> dph.hs:26:39:
>> Couldn't match expected type `[(String, String)]'
>> with actual type `(String, String)'
>> In the second argument of `map', namely `hash_tuples'
>> In the expression: map tuple_to_string hash_tuples :: [String]
>> In an equation for `strings':
>> strings = map tuple_to_string hash_tuples :: [String]
>> make: *** [dph] Error 1
>> above cmd output done 2015 Tue Dec 01 04:05:18 PM PST
>>
>>
>> dph.hs looks like:
>> import Md5s
>> import Split0
>> import System.IO
>>
>> get_filenames :: String -> [String]
>> get_filenames buffer = do
>> -- Let's hope this doesn't give locale-related roundtrip problems.
>> Split0.split0 '\0' buffer :: [String]
>>
>> do_prefix_hash :: String -> (IO String, String)
>> do_prefix_hash filename = do
>> hash <- Md5s.prefix_md5 filename :: (IO String)
>> (hash, filename)
>>
>> tuple_to_string :: (String, String) -> String
>> tuple_to_string (first, second) = do
>> (show first) ++ " " ++ (show second)
>>
>> main :: IO ()
>> main = do
>> buffer <- (System.IO.hGetContents System.IO.stdin) :: IO String
>> let filenames = (get_filenames buffer) :: [String]
>> io_hash_tuples <- map do_prefix_hash filenames :: [(IO String,
>> String)]
>> hash_tuples <- sequence io_hash_tuples :: [(String, String)]
>> let strings = map tuple_to_string hash_tuples :: [String]
>> mapM_ putStrLn strings
>>
>>
>> And Md5s.hs looks like:
>> module Md5s where
>>
>> import qualified System.IO
>> import qualified Text.Printf
>> -- cabal install cryptohash
>> import qualified Crypto.Hash.MD5
>> import qualified Data.ByteString
>> import qualified Data.ByteString.Lazy
>>
>> --
>> http://stackoverflow.com/questions/10099921/efficiently-turn-a-bytestring-into-a-hex-representation
>> byte_string_to_hex :: Data.ByteString.ByteString -> String
>> byte_string_to_hex = concatMap (Text.Printf.printf "%02x") .
>> Data.ByteString.unpack
>>
>> prefix_md5 :: String -> IO String
>> prefix_md5 filename = do
>> let prefix_length = 1024
>> file <- System.IO.openBinaryFile filename System.IO.ReadMode :: IO
>> System.IO.Handle
>> data_read <- Data.ByteString.hGet file prefix_length :: IO
>> Data.ByteString.ByteString
>> _ <- System.IO.hClose file
>> let hasher = Crypto.Hash.MD5.init :: Crypto.Hash.MD5.Ctx
>> let hasher2 = Crypto.Hash.MD5.update hasher data_read ::
>> Crypto.Hash.MD5.Ctx
>> let binary_digest = Crypto.Hash.MD5.finalize hasher2 ::
>> Data.ByteString.ByteString
>> let hex_digest = byte_string_to_hex binary_digest :: String
>> return hex_digest :: IO String
>>
>> full_md5 :: String -> IO String
>> full_md5 filename = do
>> file <- System.IO.openBinaryFile filename System.IO.ReadMode :: IO
>> System.IO.Handle
>> data_read <- Data.ByteString.Lazy.hGetContents file :: IO
>> Data.ByteString.Lazy.ByteString
>> let binary_digest = Crypto.Hash.MD5.hashlazy data_read ::
>> Data.ByteString.ByteString
>> let hex_digest = byte_string_to_hex binary_digest :: String
>> -- Does this get closed for us later?
>> -- strace shows the file getting closed without our explicit close.
>> -- _ <- System.IO.hClose file
>> return hex_digest :: IO String
>>
>>
>> It might be easier to view these at
>> http://stromberg.dnsalias.org/svn/equivalence-classes/trunk/equivs3-haskell/
>> , so the line numbers are precise.
>>
>> What is the deal?
>>
>> Can anyone tell me what should be running through my head to fix this
>> kind of problem on my own in the future?
>>
>> Thanks!
>>
>> --
>> Dan Stromberg
>>
>> _______________________________________________
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>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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--
Dan Stromberg
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