Re: [Haskell-beginners] More type errors I'm having trouble with

2 Dec 2015

      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  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-in...
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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Re: [Haskell-beginners] More type errors I'm having trouble with

David McBride