I'm trying to read something from a file, do something with it, and then write it out again. I don't know, or care about, the type of the object I'm reading, but I do know its class. Here's my code: ---------- import Data.Binary ( Binary, encode, decode ) import Data.ByteString.Lazy as B ( readFile, writeFile ) import Codec.Compression.GZip ( compress, decompress ) class ( Binary a, Show a, Eq a ) => Thing a where doSomething :: a -> IO a readThing :: ( Thing a ) => FilePath -> IO a readThing f = return . decode . decompress =<< B.readFile f writeThing :: ( Thing a ) => FilePath -> a -> IO () writeThing f = B.writeFile f . compress . encode main = do a <- readThing "file1.txt" a' <- doSomething a writeThing "file2.txt" a' ---------- And here's the compiler error I get. Amy.hs:18:3: Ambiguous type variable `a0' in the constraint: (Thing a0) arising from a use of `writeThing' Probable fix: add a type signature that fixes these type variable(s) In the expression: writeThing "file2.txt" a' In the expression: do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } Is there a way to fix my code? I tried adding a type signature, e.g., a <- readThing "file1.txt" :: (Thing t) => IO t a' <- doSomething a :: (Thing t) => IO t but then it says... Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: Thing t => IO t The following variables have types that mention t0 a :: t0 (bound at Amy.hs:16:3) In a stmt of a 'do' expression: a' <- doSomething a :: Thing t => IO t In the expression: do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' }