CODEWITHSUNDEEP
haskellindentation
darwinsenior
darwinsenior

Reputation: 496

Haskell indentation block "last statement in a do-block must be an expression"

I am very confused about haskell indentation especially do block with let and where

I got the following error on line action <- "last statement in a do-block must be an expression" How should I fix this?

I have the following definition

type NeovimRead = Neovim r st Text
type NeovimWrite = Text -> Neovim r st ()

consumeLoop :: Server -> NeovimRead -> NeovimWrite -> Neovim r st ()
consumeLoop server read write = do
    status <- liftIO $ takeMVar (status server)
    if status == Running
       then loop
       else liftIO $ putMVar (status server) status
  where loop = do
    action <- liftIO $ readaction server
    case action of
      ReadBuffer -> do
        text <- read
        liftIO $ putMVar (buffer server) text
      WriteBuffer -> do
        text <- liftIO $ takeMVar (buffer server)
        write text
    liftIO $ putMVar (status server) Running
    consumeLoop server read write

Upvotes: 1

Views: 287

Answers (1)

Will Sewell
Will Sewell

Reputation: 2643

The body of loop needs to be indented more than its name. e.g.:

consumeLoop :: Server -> NeovimRead -> NeovimWrite -> Neovim r st ()
consumeLoop server read write = do
    status <- liftIO $ takeMVar (status server)
    if status == Running
       then loop
       else liftIO $ putMVar (status server) status
  where
    loop = do
      action <- liftIO $ readaction server
      ...

Upvotes: 3

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