Appendix III: Fission Style Guide

Type Signatures
Type signatures are required
Single-Line
Right
Wrong
id :: a -> a
id x = x
id 
  :: a 
  -> a
id x = x
Multi-Line
Right
Wrong
rawList ::
  ( MonadRIO          cfg m
  , Has IPFS.BinPath  cfg
  , Has IPFS.Timeout  cfg
  , HasProcessContext cfg
  , HasLogFunc        cfg
  )
  => m (ExitCode, Lazy.ByteString, Lazy.ByteString)
rawList = IPFSProc.run' ["bootstrap", "list"]
rawList :: MonadRIO          cfg m
        => Has IPFS.BinPath  cfg
        => Has IPFS.Timeout  cfg
        => HasProcessContext cfg
        => HasLogFunc        cfg
        => m (ExitCode, Lazy.ByteString, Lazy.ByteString)
rawList = IPFSProc.run' ["bootstrap", "list"]
Why is it like this? A major reason is that the syntax highlighter in VS Code breaks with the :: on the next line. We've also reverted to using tuple-style constraints, because in practive they making distinguishing between the constraints and argument types.
Pipes
Indent multi-line pipelines by 2 from the first item. This makes it consistent in do-notation and normal notations.
Right
Wrong
app
  |> condDebug 
  |> CORS.middleware
  |> runner settings
  |> liftIO
app
|> condDebug 
|> CORS.middleware
|> runner settings
|> liftIO
As much as possible, pipe in one direction
Right
Wrong
Hku.Password <| encodeUtf8 <| Hku.password <| Hku.api <| manifest
Hku.Password <| encodeUtf8 (manifest |> Hku.api |> Hku.password)
Break long pipelines into multiple lines where possible
Right
Wrong
app
  |> condDebug
  |> CORS.middleware
  |> runner settings
  |> liftIO
​
app |> condDebug |> CORS.middleware |> runner settings |> liftIO
Do not mix pipes with the bind operator
Right
Wrong
app <- Web.app
​
app
  |> condDebug 
  |> CORS.middleware
  |> runner settings
  |> liftIO
Web.app
  >>= condDebug
  |> CORS.middleware
  |> runner settings
  |> liftIO
Avoid the Point-Free Style
Refrain from defining functions in point-free style:
Right
Wrong
docs :: Web.Host -> Swagger
docs host' =
  host'
    |> app (Proxy @Web.API)
    |> dns
    |> ipfs
    |> heroku
    |> ping
    |> user
docs :: Web.Host -> Swagger
docs = app (Proxy @Web.API)
     . dns
     . ipfs
     . heroku
     . ping
     . user
​
However, they often do read well in a pipeline:
Right
Acceptable
alphaNum :: MonadIO m => Natural -> m Text
alphaNum len =
  len
    |> bsRandomLength
    |> fmap (decodeUtf8Lenient . BS.filter isAsciiAlphaNum)
alphaNum :: MonadIO m => Natural -> m Text
alphaNum len =
  len
    |> bsRandomLength
    |> fmap (\str ->
         str 
           |> BS.filter isAsciiAlphaNum
           |> decodeUtf8Lenient)
MonadIO
Whenever possible, generalize IO functions to MonadIO. This prevents you from needing to generalize the function at the call site, while still able to use it in IO contexts.
Right
Wrong
fromHandler :: MonadIO m => Handler a -> m a
fromHandler handler =
  liftIO <| runHandler handler >>= \case
    Right inner     -> pure inner
    Left servantErr -> throwM servantErr
    
-- In use...
​
server appHost = Web.Swagger.server fromHandler appHost
            :<|> IPFS.server
            :<|> const Heroku.server
            :<|> User.server
            :<|> pure Ping.pong
            :<|> DNS.server
fromHandler :: Handler a -> IO a
fromHandler handler =
  runHandler handler >>= \case
    Right inner     -> pure inner
    Left servantErr -> throwM servantErr
​
-- In use...
​
server appHost = Web.Swagger.server (liftIO . fromHandler) appHost
            :<|> IPFS.server
            :<|> const Heroku.server
            :<|> User.server
            :<|> pure Ping.pong
            :<|> DNS.server

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Type Signatures

Single-Line

Multi-Line

Pipes

Avoid the Point-Free Style

MonadIO