[Haskell-beginners] calling inpure functions from pure code

Antoine Latter aslatter at gmail.com
Fri Oct 12 22:06:13 CEST 2012

On Fri, Oct 12, 2012 at 8:28 AM, Emmanuel Touzery <etouzery at gmail.com> wrote:
> Hi,
>> when parsing the string representing a page, you could
>> save all the links you encounter.
>> After the parsing you would load the linked pages and start
>> again parsing.
>> You would redo this until no more links are returned or a
>> maximum deepness is reached.
> Thanks for the tip. That sounds much more reasonable than what I mentioned.
> It seems a bit "spaghetti" to me though in a way (but maybe I just have to
> get used to the Haskell way).
> To be more specific about what I want to do: I want to parse TV programs. On
> the first page I have the daily listing for a channel. start/end hour,
> title, category, and link or not.
> To fully parse one TV program I can follow the link if it's present and get
> the extra info which is there (summary, pictures..).

If this were me, I would write the following:

data ChannelListing = ChannelListing [BasicProgramInfo]

-- | Summary of a program
data BasicProgramInfo =
    { basicStartTime :: ...
    , basicEndTime :: ...
    , basicTitle :: ...
    , basicUrl :: URL

-- | Full details of a program
data ProgramInfo = ...

fetchChannelListing :: ChannelId -> IO ChannelListing

fetchProgramInfo :: BasicProgramInfo -> IO ProgramInfo

And then I would string my program together from these primitives.
That way large portions of the code can be built up from the pure data
types, but the top-level can load them up as needed with impure

This is just my first impression, though.


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