Make a threadDelay-like function in Parsec - Haskell

I have been making my own Lisp-y language using Parsec in Haskell over the last few weeks, following the "Write Yourself a Scheme in 48 Hours" guide. Note that I am still a beginner at Haskell.

I'm trying to implement a "sleep" function that waits a certain time then prints "true" after the command has been inputted at the REPL, using my custom types from Parsec (I am not using a lexer/tokens). The problem is is that when I input the delay function into the REPL, it just shows "done" instead of suspending the thread. Also, I'm using "Control.Monad.Error" instead of "Control.Monad.Except" because the tutorial is using Error.

delay

Here are my types (compressed for convenience):

-- All the custom data types, which follows the syntax: [Name - Haskell Type]
data Values = Atom String
| Number Integer
| InOut (IO ())
| IOFunc ([Values] -> IOThrowsError Values) -- This is used for File and REPL IO

instance Show Values where show = showVal


This is my "show" function (compressed for convenience):

showVal :: Values -> String
showVal (InOut _) = "done"
showVal (IOFunc _) = "<primitive>"


Finally, the function:

import Control.Concurrent (threadDelay)

dTime :: [Values] -> IOThrowsError Values
dTime [Number n] =
let delay' = fromIntegral n
in (return . InOut) (threadDelay delay')


REPL output:

BuBBLE> (delay 1000) ; it works partially, but it immediately outputs `done`
done


Does threadDelay only work in GHC, or what am I missing to make it work?

Full source: Ninjacop/BuBBLE

1000

dTime

threadDelay

1 Answer
1

dTime yields a value that contains an IO action which, if you executed it, would suspend the thread. Good. But showVal never attemps executing any IO – it just sees, “there is an IO action here”, but doesn't do anything with it. In fact it can't do anything with it, due to the pure signature -> String. To make it eval too, you'll probably want to use evalString :: Env -> String -> IO String or something instead.

dTime

IO

showVal

IO

IO

-> String

evalString :: Env -> String -> IO String

evalString

dTime

(delay 1000)

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