Question about error message

[Paulo Sequeira]

Ok, you got me.

My wording was not careful, so I must rectify.

If you try

    let xyz = do getLine in do p <- xyz; putStr p

you'll see that in xyz I'm not really discarding the string read (you caught
it later in p).

Definitively, Alastair's explanation is more correct than mine and it didn't
lead you to a misunderstanding.

You must understand that do notation is mostly syntactic sugar. It is more
familiar to write

    do p <- getLine; putStr p

than to write

    getLine >>= (\ p -> putStr p)

What I really mean by "discarding" was that, if you write

   do getLine; stmt

you would simply won't have at hand the line read in stmt because the value
was not bound to any variable. Note that the previous line is equivalent to

    getLine >> do stmt

or, by the definition of >>

    getLine >>= (\ _ -> do stmt)

See it?

Also, note that when you write

    do getLine

in GHCi, it is GHCi who is "discarding" the string. (mmm, would this last
statement be also misleading?)

Hope I'm now explaining myself better.

Paulo.

----- Original Message -----
From: Seth Kurtzberg <seth@cql.com>
To: Paulo Sequeira <otro_paulo@hotmail.com>
Sent: Saturday, August 17, 2002 7:25 PM
Subject: Re: Question about error message


> Paulo, and also Alastair,
>
> Thanks for the responses.  And, from a practical perspective, Paulo's
> workaround is interesting (and also works).  But ...
>
> Wait a minute.  The type (according to GHCI) of both:
>
>     xyz = do getLine
>
> and
>
>     abc = do p <- getLine
>                   return p
>
> is IO String.  And, in fact, both expressions indeed return the String
part of
> the IO String.  So (I'm playing devil's advocate here) you are asserting
that
> it is correct for one entity of type IO String to provoke different
behavior
> than another entity of type IO String.  All "IO String"s are not created
> equal.
>
> I don't see, fundamentally, how it violates referential integrity or any
other
> functional programming principle to stick a value somewhere and not use
it.
> It might not be very intelligent, or useful, but that is irrelevant from
the
> theoretical perspective.
>
> And, the restriction has a negative impact on code reuse.  I shouldn't
need to
> define both "abc" and "xyz" in the above example, which do the same thing,
> merely because sometimes I want the result and other times I don't.
>
> On Saturday 17 August 2002 16:14, you wrote:
> > Perhaps it will help if you recall the meaning of the do notation,
> > according to the Haskell98 Report.
> >
> >     do {e} = e
> >     do {e;stmts} = e >> do {stmts}
> >     do {p <- e; stmts} =
> >         let
> >             ok p = do {stmts}
> >             ok _ = fail "..."
> >         in e >>= ok
> >     do {let decls; stmts} = let decls in do {stmts}
> >
> >
> > Note that "do p <-e" only has meaning when is followed by something else
> > (e.g. an expresion). One way to explain it is that it only makes sense
to
> > introduce "p" to use it later (e.g. you are reading a string and binding
it
> > to "p" to parse it later). If you are only interested in the side
effects
> > of "getLine", you should write:
> >
> > main = do getLine
> >
> > This will effectively read a line and discard it afterwards. Try it.
> >
> > Paulo.
> >
> > ----- Original Message -----
> > From: Seth Kurtzberg <seth@cql.com>
> > To: <glasgow-haskell-users@haskell.org>
> > Sent: Saturday, August 17, 2002 2:23 PM
> > Subject: Question about error message
> >
> > > Hello, all
> > >
> > > There is an error message that I get from GHCI that I do not
understand.
> > > Usually it is caused by a syntax error or an indentation error, but I
> > > have occasionally seen it in other contexts and I'd like to understand
> > > what it
> >
> > is
> >
> > > trying to tell me.
> > >
> > > The message is that the last statement in a do construct must be an
> >
> > expression
> >
> > > (exact text below).
> > >
> > > I believe this occurs when the last statement in the do construct has
a
> >
> > type
> >
> > > other than "IO ()".  The documentation (or perhaps Simon Thompson's or
> >
> > Paul
> >
> > > Hudak's book; they get jumbled together in my mind) states that all
> > > statements in a do construct have type "IO ()"; but this is clearly
not
> > > correct, as in:
> > >
> > >     do line <- getLine
> > >         putStrLn line
> > >
> > > where the first statement has type IO String.
> > >
> > > Here is an example.  This code is useless but it is not obvious to me
why
> >
> > it
> >
> > > is incorrect:
> > >
> > >     main = do line <- getLine
> > >
> > > producing the error:
> > >
> > > <interactive>:1: The last statement in a 'do' construct must be an
> >
> > expression
> >
> > > (This also limits the ability to enter monadic programming to the
> > > interpreter.)
> > >
> > > TIA,
> > >
> > > --
> > > Seth Kurtzberg
> > > M. I. S. Corp
> > > seth@cql.com
> > > 1-480-661-1849 (GMT-7)
> > >
> > > _______________________________________________
> > > Glasgow-haskell-users mailing list
> > > Glasgow-haskell-users@haskell.org
> > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>
> --
> Seth Kurtzberg
> M. I. S. Corp
> seth@cql.com
> 1-480-661-1849 (GMT-7)
>