I’m not sure exactly how much this falls under ‘programming’ opposed to ‘program language design’. But the issue is this:
Say, for sake of simplicity we have two ‘special’ lists/arrays/vectors/whatever we just call ‘ports’ for simplicity, one called stdIn and another stdOut. These conceptually represent respectively
- All the user input given to the program in the duration of the program
- All the output written to the terminal during the duration of the program
In Haskell-inspired pseudocode, it should then be possible to create this wholly declarative program:
let stdOut = ["please input a number",
"and please input another number",
"The product of both numbers is: " ++ stdIn[0] * stdIn[1]]
Which would do the expected, ask for two numbers, and print their product. The trick being that stdOut represents the list of strings written to the terminal at the completion of the program, and stdIn the list of input strings. Type errors and the fact that there needs to be some safeguard to only print the next line after a new line has been entered left aside here for simplicity’s sake, it’s probably easy enough to solve that.
So, before I go of to implement this idea, are there any pitfalls to it that I overlooked? I’m not aware of a similar construct already existing so it’d be naïve to not take into account that there is an obvious pitfall to it I overlooked.
Otherwise, I know that of course:
let stdOut = [stdIn[50],"Hello, World!"]
Would be an error if these results need to be interwoven in a similar fashion as above.
A similar approach was used in early versions of Haskell, except that the elements of the stdin and stdout channels were not strings but generic IO ‘actions’–in fact, input and output were generalized to ‘response’ and ‘request’. As long as both channels are lazy (i.e. they are actually ‘iterators’ or ‘enumerators’), the runtime can simply walk the request channel, act on each request and tack appropriate responses onto the response channel. Unfortunately, the system was very hard to use, so it was scrapped in favor of monadic IO. See these papers: