haskell - No instance for Show arising from a use in "main" level -

i have code reads files , parses using uu.parsing lib returns abstract sintax tree , shows on screen.

i received error message "no instance show" in functions originated in tokensparsertobytestring , applyparser using parseio (of uu.parsing lib) , inherited signatures until main. fixed signatures problem in main function. added instance show in signature have next compilation error:

no instance (show (io j2s)) arising use of ‘main’ in expression: main when checking type of io action ‘main’ 

the complete error message is:

$ cabal build building java2scala-1.0... preprocessing library java2scala-1.0... in-place registering java2scala-1.0... preprocessing executable 'java2scala' java2scala-1.0... preprocessing executable 'test' java2scala-1.0... [5 of 5] compiling main             ( test/main.hs, dist/build/test/test-tmp/main.o )  test/main.hs:27:1:     no instance (show (io j2s)) arising use of ‘main’     in expression: main     when checking type of io action ‘main’ 

some idea, problem?

main module

{-# language flexiblecontexts #-} module main  import uu.parsing ... import content  main :: (show (io j2s)) => io() main = f <- getline       let command = test f       command  test :: (show (io j2s)) => string -> io() test "testparser" = testparser 

test module

{-# language flexiblecontexts #-} module j2s.parser.test  import content import j2s.ast.sintax import j2s.parser import uu.parsing ...  testparser :: (show (io j2s)) => io() testparser  = (runsafeio $ runproxy $ runeitherk $                 contentsrecursive "path/of/my/tests" />/ handlerparser) :: (show (io j2s)) => io() 

content module

{-# language flexiblecontexts #-} module content  import control.monad(form, liftm) import system.directory (doesdirectoryexist, getdirectorycontents) import system.filepath ((</>), splitextension, splitfilename) import j2s.parser import j2s.ast.sintax import uu.parsing  import control.monad (when, unless) import control.proxy import control.proxy.safe hiding (readfiles)  import j2s.scanner.token import text.show  import uu.parsing   contentsrecursive      :: (checkp p)      => filepath -> () -> producer (exceptionp p) filepath safeio () contentsrecursive path () = loop path         loop path =          contents path () //> \newpath ->              respond newpath              isdir <- tryio $ doesdirectoryexist newpath              let ischild = not $ takefilename newpath `elem` [".", ".."]              when (isdir && ischild) $ loop newpath   applyparser :: (proxy p, show (io j2s)) => string -> consumer p b.bytestring io () applyparser path = runidentityp loop       loop =         bs <- request ()         let sc = classify  (initpos path) (b8.unpack bs)         lift $  b8.putstrln (tokensparsertobytestring sc)  tokensparsertobytestring :: (show (io j2s)) =>  [token] -> b.bytestring tokensparsertobytestring tokens = b8.pack(show (parseio pj2s tokens))   handlerparser :: (checkp p, show (io j2s)) => filepath -> session (exceptionp p) safeio () handlerparser path =     canread <- tryio $ fmap readable $ getpermissions path     isdir   <- tryio $ doesdirectoryexist path     isvalidextension <- tryio $ evaluate ((snd (splitextension path) == ".java" || snd (splitextension path) == ".mora") && (snd (splitfilename path) /= "encodetest.java") && (snd (splitfilename path) /= "t6302184.java") && (snd (splitfilename path) /= "unmappable.java"))     when (not isdir && canread && isvalidextension) $         (readfilesp 10240 path >-> try . applyparser) path   readfilesp :: (checkp p) => int -> filepath -> () -> producer (exceptionp p) b.bytestring safeio () readfilesp chunksize path () =     bracket id (openfile path readmode) hclose $ \handle ->         let loop =                 eof <- tryio $ hiseof handle                 unless eof $                     bs <- tryio $ b.hgetsome handle chunksize                     respond bs                     loop         loop 

a signature show (io j2s) => io () never makes sense. expresses “provided universe crafted such io j2s has show instance, give io () action”. well, if universe has property, give io () action right now. keep nasty chipsconstraints!
constraints make sense if apply them type variables, i.e. if you're writing code that's polymorphic on several different, not all types. (like checkp p). constraint applied concrete types little more defer type errors.

io j2s has no show instance. , can't have such instance: io action. complete subprogram might execute costly computations, call commercial third-party library code, launch missiles... , in end return j2s value. how expect pack information of possibly complex simple string?

what possibly have show instance j2s. if you're in io monad anyway , have io j2s action, can @ point fetch j2s value monad-binding action (i.e. executing subprogram) , showing j2s value. in case:

tokensparsertobytestring :: [token] -> io b.bytestring tokensparsertobytestring tokens = fmap (b8.pack . show) $ parseio pj2s tokens 

i case you're confused fmapping in io functor, equivalent to

tokensparsertobytestring :: [token] -> io b.bytestring tokensparsertobytestring tokens =      j2svalue <- parseio pj2s tokens      return . b8.pack $ show j2svalue 

of course need adapt applyparser because tokensparsertobytestring io action. easy enough =<< operator:

applyparser :: proxy p => string -> consumer p b.bytestring io () applyparser path = runidentityp loop       loop =         bs <- request ()         let sc = classify  (initpos path) (b8.unpack bs)         lift $ b8.putstrln =<< tokensparsertobytestring sc