[Haskell-beginners] More type errors I'm having trouble with
Simon Jakobi
simon.jakobi at googlemail.com
Wed Dec 2 03:47:48 UTC 2015
Hi Dan,
I'm having a hard time understanding those error messages too. It
seems to me that part of the problem is that GHC is confused by some
incorrect type annotations of yours.
I suggest that you delete or at least comment out your own type
annotations and then either work with the hopefully simpler error-messages
from GHC or use the `:load` and `:type` commands in ghci to discover
the inferred
types for your functions.
Depending on how close the inferred type is to the intended type, you
may have to adjust the definitions.
Also note that you sometimes incorrectly and unnecessarily use
do-notation in "non-monadic"/plain functions, for example in
tuple_to_string and get_filenames.
You could also consider following a book or a course until you feel
more comfortable trying things on your own:
Take a look at learnyouahaskell.com or github.com/bitemyapp/learnhaskell.
Good luck!
Simon
2015-12-02 1:12 GMT+01:00, Dan Stromberg <strombrg at gmail.com>:
> 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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