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

[Vale Cofer-Shabica]

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 at gmail.com
401.267.8253


On Tue, Aug 26, 2014 at 2:23 PM, Brandon Allbery <allbery.b at gmail.com> wrote:
> On Tue, Aug 26, 2014 at 2:16 PM, Vale Cofer-Shabica
> <vale.cofershabica at gmail.com> 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 at gmail.com                                  ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
>
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