Thanks guys, I understand what you're telling me, but have some nested IFs and just want to fall through on one of the ELSES but then I end up with two ELSES in a row and nothing between them. Oh, well, on to restructuring. Michael --- On Wed, 10/21/09, Tim Wawrzynczak wrote: From: Tim Wawrzynczak Subject: Re: [Haskell-cafe] Is there a null statement that does nothing? To: "michael rice" Cc: haskell-cafe@haskell.org Date: Wednesday, October 21, 2009, 8:49 PM Yes, an if statement must have both 'then' and 'else' branches.  As an example, what if you had let a = if b == 2 then True else False and you were missing an else branch?  What would 'a' get assigned to? The if statement "returns" a value so must have both branches. However, in a monadic constraint, there are the functions 'when' and 'unless.'  They allow conditional evaluation of expressions in a monadic context.  For example, main = do   line <- getLine   when (line == "hello") putStrLn "Hello back!" Cheers,  - Tim On Wed, Oct 21, 2009 at 7:43 PM, michael rice wrote: It looks like both the THEN and the ELSE in an IF expression must each have an expression. What's a graceful way to do nothing in either or both slots, kind of like the Fortran CONTINUE statement.   --mr ================ [michael@localhost ~]$ ghci GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> if (1==1) then else <interactive>:1:15: parse error on input `else' Prelude> if (1==1) then True else <interactive>:1:24: parse error (possibly incorrect indentation) Prelude> if (1==1) then True else False True Prelude> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Those nested IF/THEN/ELSEs are real killers. I kind of use them to sort out my thoughts, then end up with a nested mess. The restructuring to WHENs went smoothly and looks a lot cleaner besides. Michael --- On Wed, 10/21/09, Dan Weston wrote: From: Dan Weston Subject: Re: [Haskell-cafe] Is there a null statement that does nothing? To: "michael rice" Cc: "Tim Wawrzynczak" , "haskell-cafe@haskell.org" Date: Wednesday, October 21, 2009, 9:16 PM If you have a long if/else if/else chain, you might consider a trivial case statement with guards. Whether you think this is attractive is a matter of taste, but it has the fall-through semantics you want and ghc optimizes the _ pattern matching away: f x = case () of   _| x == 2    -> 22   _| x == 4    -> 44   _| x == 7    -> 77   _| otherwise -> 55

f 4 44 f 9 55

michael rice wrote:

Thanks guys,

I understand what you're telling me, but have some nested IFs and just want to fall through on one of the ELSES but then I end up with two ELSES in a row and nothing between them. Oh, well, on to restructuring.

Michael

--- On *Wed, 10/21/09, Tim Wawrzynczak //* wrote:

     From: Tim Wawrzynczak      Subject: Re: [Haskell-cafe] Is there a null statement that does nothing?      To: "michael rice"      Cc: haskell-cafe@haskell.org      Date: Wednesday, October 21, 2009, 8:49 PM

     Yes, an if statement must have both 'then' and 'else' branches.  As      an example, what if you had

     let a = if b == 2 then True else False

     and you were missing an else branch?  What would 'a' get assigned to?

     The if statement "returns" a value so must have both branches.

     However, in a monadic constraint, there are the functions 'when' and      'unless.'  They allow conditional evaluation of expressions in a      monadic context.  For example,

     main = do        line <- getLine        when (line == "hello") putStrLn "Hello back!"

     Cheers,       - Tim

     On Wed, Oct 21, 2009 at 7:43 PM, michael rice > wrote:

         It looks like both the THEN and the ELSE in an IF expression          must each have an expression. What's a graceful way to do          nothing in either or both slots, kind of like the Fortran          CONTINUE statement.

           --mr

         ================

         [michael@localhost ~]$ ghci          GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help          Loading package ghc-prim ... linking ... done.          Loading package integer ... linking ... done.          Loading package base ... linking ... done.          Prelude> if (1==1) then else

         <interactive>:1:15: parse error on input `else'          Prelude> if (1==1) then True else

         <interactive>:1:24: parse error (possibly incorrect indentation)          Prelude> if (1==1) then True else False          True          Prelude>

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Hi Curt, Thanks for the tips. Looks like return () would be a good candidate for that null statement I was seeking earlier. Michael --- On Sun, 10/25/09, Curt Sampson wrote: From: Curt Sampson Subject: Re: [Haskell-cafe] Is there a null statement that does nothing? To: "michael rice" Cc: "haskell-cafe@haskell.org" Date: Sunday, October 25, 2009, 12:08 AM On 2009-10-21 18:27 -0700 (Wed), michael rice wrote:

Those nested IF/THEN/ELSEs are real killers. I kind of use them to sort out my thoughts, then end up with a nested mess. The restructuring to WHENs went smoothly and looks a lot cleaner besides.

Indeed. Monadic control flow is one of the basic keys to being able to write nice code in Haskell; it's the moral equivalant of using polymorphism rather than than "case" or "switch" in OO languages. (Well, some would disagree, but that gets into the whole OO vs. functional debate.) You'll probably want to spend some time with one of the zillion monad tutorials at some point (I like "All About Monads") to learn more about these things. BTW, look up the source code for "when". :-)   http://haskell.org/ghc/docs/latest/html/libraries/base/src/Control-Monad.htm... cjs -- Curt Sampson               +81 90 7737 2974            Functional programming in all senses of the word:                    http://www.starling-software.com

Hello michael, Thursday, October 22, 2009, 4:59:43 AM, you wrote: "return ()" does the trick if another branch also returns ()

Thanks guys,

I understand what you're telling me, but have some nested IFs and just want to fall through on one of the ELSES but then I end up with two ELSES in a row and nothing between them. Oh, well, on to restructuring.

Michael

--- On Wed, 10/21/09, Tim Wawrzynczak wrote:

From: Tim Wawrzynczak Subject: Re: [Haskell-cafe] Is there a null statement that does nothing? To: "michael rice" Cc: haskell-cafe@haskell.org Date: Wednesday, October 21, 2009, 8:49 PM

Yes, an if statement must have both 'then' and 'else' branches.  As an example, what if you had

let a = if b == 2 then True else False

and you were missing an else branch?  What would 'a' get assigned to?

The if statement "returns" a value so must have both branches.

However, in a monadic constraint, there are the functions 'when' and 'unless.'  They allow conditional evaluation of expressions in a monadic context.  For example,

main = do   line <- getLine   when (line == "hello") putStrLn "Hello back!"

Cheers,  - Tim

On Wed, Oct 21, 2009 at 7:43 PM, michael rice wrote:

It looks like both the THEN and the ELSE in an IF expression must each have an expression. What's a graceful way to do nothing in either or both slots, kind of like the Fortran CONTINUE statement.

  --mr

================

[michael@localhost ~]$ ghci GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude>> if (1==1) then else

<interactive>:1:15: parse error on input `else' Prelude>> if (1==1) then True else

<interactive>:1:24: parse error (possibly incorrect indentation) Prelude>> if (1==1) then True else False True Prelude>>

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-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

Hi Brent, Re: '"if" expressions, not statements' is an elegant clarification, one definitely for the haskellwiki, if not there already. Just for completeness' sake, bottom is a value for any expression. Wouldn't making the else clause optional by defaulting to "undefined" worthy of consideration for Evil Haskell? -- Kim-Ee Brent Yorgey-2 wrote:

On Wed, Oct 21, 2009 at 07:49:14PM -0500, Tim Wawrzynczak wrote:

...

Yes, an if statement must have both 'then' and 'else' branches. As an example, what if you had

let a = if b == 2 then True else False

and you were missing an else branch? What would 'a' get assigned to?

The if statement "returns" a value so must have both branches.

By the way, it helps to be precise with language here: "statement" usually indicates an instruction which causes some effect to happen. "Expression" indicates something which evaluates to a value. Haskell doesn't have "if statements" but it does have "if expressions". It makes sense for an if *statement* (in imperative languages like C) to have an optional else clause, since "do nothing" is a perfectly valid *statement*. But it doesn't make sense for an if *expression* to have a missing else, since it must evaluate to something and (in general) there is no "null value" that could be used. Of course, as others have noted, in a monadic context there IS a special "null value", namely return (), which can be used to indicate "do nothing".

-Brent _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- View this message in context: http://www.nabble.com/Is-there-a-null-statement-that-does-nothing--tp2600244... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

Hello Kim-Ee, Tuesday, October 27, 2009, 8:08:27 PM, you wrote:

Just for completeness' sake, bottom is a value for any expression. Wouldn't making the else clause optional by defaulting to "undefined" worthy of consideration for Evil Haskell?

in this case you will get an exception when condition is false (when 'if' used in expression) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com