[Haskell-beginners] text file to data type

[Joe Fredette]

My suggestion would be to look into writing a parser (via parsec) to  
handle this. Parsec is fairly easy to learn, and since your data is a  
pretty simple format, the parser won't be hard to write. Parsec will  
then give you a parser which you can run on the file, it'll catch  
parse errors, it's all around very lovely to use. There is a chapter  
of Real World Haskell on the subject, and I'm sure we'll be happy to  
help with whatever isn't covered.

/Joe


On Dec 7, 2009, at 10:43 PM, Brent Pedersen wrote:

> hi, i have files with lots (millions of rows) of data like this:
> chr6    chr10  96.96   3392    101 2   79030508    79033899    4160024
> 4163413 0.0 5894
> chr6    chr10  93.19   4098    228 13  117152751   117156826
> 11355389    11359457    0.0 5886
> chr6    chr10  95.82   3445    130 5   112422073   112425513   7785396
> 7788830 0.0 5666
>
> and i'd like to read it into a type like this:
>
> data Blast = Blast { query :: S.ByteString
>                   , subject :: S.ByteString
>                   , hitlen :: Int
>                   , mismatch :: Int
>                   , gaps :: Int
>                   , qstart :: Int
>                   , qstop :: Int
>                   , sstart :: Int
>                   , sstop :: Int
>                   , pctid :: Double
>                   , evalue :: Double
>                   , bitscore :: Double
>                   } deriving (Show)
>
> where each of those fields corresponds to a column in the file.
> in python, i do something like:
>
>>>> line = [fn(col) for fn, col in zip([str, str, int, int, int, int,  
>>>> int, int, int, float, float], sline.split("\t")]
>
> what's a fast, simple way to do this in haskell?
>
> is it something like:
>
> instance Read Blast where
>    readsPrec s = ?????
>
>
> any pointers on where to look for simple examples of this type of  
> parsing would
> be much appreciated.
> thanks,
> -brentp
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