Re: [Haskell-beginners] do notation, pattern matching, if, & case

Thank you! I was able to re-write the case-of version as well so it compiles too. I'll read up on de-sugaring do-notation. -vale -- vale cofer-shabica vale.cofershabica@gmail.com 401.267.8253 On Tue, Aug 26, 2014 at 2:23 PM, Brandon Allbery wrote:

On Tue, Aug 26, 2014 at 2:16 PM, Vale Cofer-Shabica wrote:

...

However, if I try to use f' or f'', I get parse errors. Three questions:

* Is there a better way of doing this * Are stylistic changes to f really called for? * How can/ought I correct my syntax errors?

if and case are expressions. As such they are not part of the outer "do"'s syntax; you would need a new "do" in each one. This will make more sense if you study how "do" is converted to uses of the (>>) and (>>=) operators.

Additionally you can't just use "s <- getContents" like that, since (a) "s" will go out of scope, abnd (b) with nothing following, it expands to a syntax error ("getContents >>= \s ->" with nothing after the "->"). Since if and case are expressions, you need to produce a result from them and bind it at the top level. So you really want something like:

f' :: String -> IO () f' file = do s <- if file == "-" then getContents else readFile file putStrLn s

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners