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

[David Feuer]

If-then-else is a syntactic convenience. Missing cases are usually wrong.
We don't want to go out of our way to make it convenient to write wrong
code.

On Mon, Jul 9, 2018, 1:20 PM Johannes Waldmann <
johannes.waldmann at htwk-leipzig.de> 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 }"
>
> 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#SECTION001160000000000000000
> (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
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