haskell_testing.hs

haskell_testing.hs

import qualified Prelude
import Prelude hiding(readFile)
import Control.Monad.State

class Monad m => FSMonad m where
  readF :: FilePath -> m String

numCharactersInFile :: FSMonad m => FilePath -> m Int
numCharactersInFile fileName = do
    contents <- readF fileName
    return (length contents)

instance FSMonad IO where
    readF = Prelude.readFile

data MockFS = SingleFile FilePath String

newtype StateMockFS = State MockFS

instance FSMonad StateMockFS where
    readF pathRequested = do
        (SingleFile pathExisting contents) <- get
        if pathExisting == pathRequested
            then return contents
            else fail "file not found"

testNumCharactersInFile :: Bool
testNumCharactersInFile = evalState
                                (numCharactersInFile "test.txt")
                                (SingleFile "test.txt" "hello world")
                             == 11

This are couple of things that are wrong here.
The most basic is that you forgot to give the name to the newtype constructor. We can choose to call it StateMockFS too:

newtype StateMockFS = StateMockFS (State MockFS)

That's not right either because StateMockFS needs to be a type constructor, not a simple type:

newtype StateMockFS a = StateMockFS (State MockFS a)

That get us much closer, but now the compiler will ask us to implement the Monad, Applicative and Functor instances for StateMockFS, which you'll have to write (or derive using some of the language extensions).

Hi, Thanks again for the response; I had done that (all the way through adding the "a" type variable to make the newtype's kind be * -> *), but then I saw that it indeed required me to provide instances for all those type classes and I disregarded the option altogether; and the solution to this would be to add yet more extensions, which defeated the purpose, but thank you for sharing the knowledge, at least I was on the right track. :-) Milad

…

On Wed, 4 Sep 2019 at 01:39, Rotsor ***@***.***> wrote: This are couple of things that are wrong here. The most basic is that you forgot to give the name to the newtype constructor. We can choose to call it StateMockFS too: newtype StateMockFS = StateMockFS (State MockFS) That's not right either because StateMockFS needs to be a type constructor, not a simple type: newtype StateMockFS a = StateMockFS (State MockFS a) That get us much closer, but now the compiler will ask us to implement the Monad, Applicative and Functor instances for StateMockFS, which you'll have to write (or derive using some of the language extensions). — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://gist.github.com/e039b38d1dcb24f4234da79d8780fe58?email_source=notifications&email_token=AA24YYCXRH46KBRJNFYHYUTQH3YR7A5CNFSM4ITL2OV2YY3PNVWWK3TUL52HS4DFVNDWS43UINXW23LFNZ2KUY3PNVWWK3TUL5UWJTQAFYCOQ#gistcomment-3015912>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AA24YYCSU7HEGHSB45UZPJ3QH3YR7ANCNFSM4ITL2OVQ> .