[Haskell-cafe] Is there a null statement that does nothing?

[Kim-Ee Yeoh]

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
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