-- a component
data FileSystem m = FileSystem {
  exists   :: Path -> m (Either Text Bool)
, readFile :: Path -> m (Either Text Text)
}

-- a constructor for a mock file system
newFileSystemWith :: (FileSystemValues -> FileSystemValues) -> FileSystem
newFileSystemWith f = FileSystem {
  exists = \_ -> pure (_exists $ f fileSystemValues)
, readFile = \_ -> pure (_readFile $ f fileSystemValues)
}

-- mocked returned values
data FileSystemValues = FileSystemValues {
  _exists :: Either Text Bool
, _read   :: Either Text Text
}

fileSystemValues = FileSystemValues
  (Left "todo - exists")
  (Left "todo - readFile")

-- set mock values
existingFiles v = v { _exists = Right True }
someText t v = v { _read = Right t }

-- create a mock and compose functions to set return values
myMock = newFileSystemWith (existingFiles . someText "hello world")