[Haskell-cafe] if-then without -else?

[arjenvanweelden at gmail.com]

On Mon, 2018-07-09 at 19:20 +0200, Johannes Waldmann wrote:
> Dear Cafe,
> 
> a student in my class detected
> this inconsistency in the design of Haskell:
> 
> > Why require that each "if-then" has an "else"
> > while still allowing incomplete sets of patterns?
> > We could define "if p then a"
> > by translating to "case p of { True -> a }"

This is the same as (case p of { True -> a; _ -> throw
PatternDoesNotMatchException }). Therefore (if False then a) would give
an exception.

Normally, every expression has a value. The value of (if p then x else
y) is properly defined. What would be the value of (if p then x)? If it
is (), then x should also have type (), I think.

You can do something like that in a Monad, and the function is called
when. And there is an unless, which is an if-else without a then.

> I think that "but then we'd have the dangling-else problem"
> is not a good answer because this is really about semantics,
> not surface syntax.
> 
> For reference, Scheme has short-if, see
> http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-14.html#node_sec_11.4.3
> (value is "unspecified" if condition is false)
> and Common LISP does as well
> https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node84.html#SECTION001
> 160000000000000000
> (value is "nil")
> 
> Was this considered at some point in Haskell's design process?
> There is Section 5.2 on pattern matching in
> http://haskell.cs.yale.edu/wp-content/uploads/2011/02/history.pdf (*)
> but it does not mention if-then(-else).
> 
> - J.W.
> 
> (*) can we please add this to https://www.haskell.org/documentation
> Upvote this: https://github.com/haskell-infra/hl/issues/86
> _______________________________________________
> 

Hope this helps and kind regards, Arjen