[Haskell-cafe] (no subject)

staafmeister g.c.stavenga at uu.nl
Fri Aug 21 19:23:25 EDT 2009

Don Stewart-2 wrote:
> G.C.Stavenga:
>> Hi, I'm just started to learn Haskell. Coming from a programming contest
>> background (where it is important to be able to solve problems in a small
>> amount of code) I'm wondering what the best way is for simple IO.
>> A typical input file (in a programming contest) is just a bunch of
>> numbers
>> which you want to read one by one (sometimes interspersed with strings).
>> In
>> C/C++ this is easily done with either scanf or cin which reads data
>> separated by spaces. In Haskell I have not found an equally
>> satisfactionary
>> method. The methods I know of
>> 1) Stay in the IO monad and write your own readInt readString functions.
>> A lot
>> of code for something easy.
>> 2) Use interact together with words and put the list of lexemes in a
>> State
>> monad and define getInt where at least you can use read.
>> 3) Use ByteString.Char8 which has readInt (but I couldn't find a
>> readString). But one has to put it also in a State monad.
>> I think that there must be standard function that can do this. What do
>> experienced Haskellers use?
>     map read . lines
> Thank you for the reply. But this only works for if you read only integers
> all on different lines.
> But in general you have a structure like
> first line -- integer specifying the number of testcases (n)
> Then for each testcase 
> a line with an integer specifying the number of edges (e)
> a line with e pairs of string s and int p where p is the number asociated
> with string s, etc.
> Such a structure cannot be parsed by map read.lines
> What I used is "words" to tokenize and put the list in a State monad with
> readInt, readString, etc. functions, to mimic
> C code. This seems to be a lot of overkill, so there must be an simpler
> way
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