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iosynch
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1995-02-14
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-- This file contains the example program from section 7.7 of the Haskell
-- report (version 1.1) for a program using synchronisation.
main :: Dialogue
main = readChan stdin abort (\userInput -> readNums (lines userInput))
readNums :: [String] -> Dialogue
readNums inputLines = readIntP "Enter first number: " inputLines
(\num1 inputLines1 ->
readIntP "Enter second number: " inputLines1
(\num2 _ -> reportResult num1 num2))
reportResult :: Int -> Int -> Dialogue
reportResult num1 num2
= appendChan stdout ("Their sum is: "++ show (num1 + num2)) abort done
-- readIntP prints a prompt and then reads a line of input. If the
-- line contains an integer, the value of the integer is passed to the
-- success continuation. If a line cannot be parsed as an integer,
-- an error message is printed and the user is asked to try again.
-- If EOF is detected, the program is aborted.
readIntP :: String -> [String] -> (Int -> [String] -> Dialogue) -> Dialogue
readIntP prompt inputLines succ
= appendChan stdout prompt abort
(case inputLines of
(l1 : rest) -> case (reads l1) of
[(n,"")] -> succ n rest
_ -> appendChan stdout
"Error - retype the number\n" abort
(readIntP prompt rest succ)
_ -> appendChan stdout "Early EOF" abort done)