[Haskell-beginners] A basic misunderstanding of how to program
ml at isaac.cedarswampstudios.org
Sun May 9 20:12:34 EDT 2010
On 05/09/10 19:30, Ken Overton wrote:
>> I think, yes your function looks close to typical Haskell, you're just
>> missing a "do", a pair of parentheses, and an "IO":
> Sorry, I'd definitely intended the 'do' to be there.
>> interact :: String -> IO Resp
> Thanks Isaac; so after I've called this how do I get the Resp value back out of the returned IO?
By using it in some other piece of IO. For example,
main :: IO ()
main = do
resp <- interact "hi there!"
let modifiedResp = some function involving resp
You see, "IO" lets you distinguish between functions/values that require
IO actions, and those that are completely pure (always give the same
result, given the same arguments, as is typical in mathematics). Just
use your patience! : Every function that uses that resulting Resp will
have to be in IO. This is not as bad as it seems, because most of the
processing of the Resp can be pure again: for example, the way you call
parseResp which is a pure function, or "some function involving resp" in
my example above.
1. Practice. The typechecker will tell you when you get it wrong.
2. Generally try to mark less of your code as "IO", because this is good
style ( -- it turns out to be easier to code [and refactor code] with
pure functions, because you know they don't have side-effects, so you
mostly don't need to worry about "when" or "how often" they're called.)
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