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

Albert Y. C. Lai trebla at vex.net
Tue Jul 17 02:49:41 UTC 2018

On 2018-07-09 01:20 PM, Johannes Waldmann wrote:
> 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.

Anthony Clayden is right that if-then-else has the benefit of giving an 
unambiguous grammar.

if-then can be made unambiguous if you throw in delimiting, e.g., 
if-then-fi, if-then-begin-end.

Suppose you like to support if-then (and have your disambiguation story) 
on the ground that case-of-True is already supported, and you don't mind 
the partiality, then why stop there?

Why is if-else getting no love from you?  Afterall, case-of-False is 
already supported, too.


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