Lazy streams and unsafeInterleaveIO

Sun, 22 Dec 2002 15:46:08 -0800

Remi Turk wrote:

>On Sun, Dec 22, 2002 at 04:00:45AM -0800, Jyrinx wrote:
>>As an experiment for a bigger project, I cooked up a simple program: It 
>>asks for integers interactively, and after each input, it spits out the 
>>running total. The wrinkle is that the function for calculating the 
>>total should be a non-monadic stream function (that is, type [Integer] 
>>-> [Integer] so that runningTotals [1,2,3,4,5] == [1,3,6,10,15]). The 
>>task is then to write a function to return a stream of integers, 
>>grabbing them from IO-land lazily (a la getContents).
>what about
>module Main where
>main    = getContents >>= mapM_ print . scanl1 (+) . map read . lines
Ooh, neat! :-) (I love these one-liners - Haskell is absurdly concise 
:-D ) Hrm ... wasn't aware of the scanl1 thingie; looks like I 
reinvented the wheel a little ... (Come to think of it, is there any 
sort of handy quick-reference card for all these combinators? Seems like 
I and other novices could stand to save some typing ...)

One sticking point, though (and this is relevant to the bigger project): 
I'd like to print a prompt somehow before each input, which I'm not sure 
is possible if I just slurp up everything from getContents ... I've 
thought of using interact somehow, but I'm not sure where I'd start with 
that one ...

(Out of curiosity: How is the compiler deciding on a type for the input? 
(That is, how does it know we want integers? Is it just a default?) 
Looks to me like all it can infer is that it's of classes Read, Show, 
and Num ... that doesn't much narrow things down ...)

BTW, I already found a major problem with the code I attached earlier, 
using unsafeInterleaveIO: Run in GHCi (as I had done), it works fine; 
but compiled by GHC and run as an executable, it waits for input and 
*then* displays the prompt after the user hits Enter ... not very 
helpful. I didn't think it would do that, since (putStr "? " >> readLn) 
seemed pretty explicit as to order of evaluation, but I guess that's 
what I get for breaking referential transparency ...

Luke Maurer