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

[Anthony Clayden]

>* Why require that each "if-then" has an "else"*


Never mind the semantics, this is a piece of syntactic history.

ALGOL 60 allowed omitted "else". That gave ambiguity:

    if p
       then if q
               then ...
           ...
           else ...

There was no layout rule. Does the "else" match to the outer "if-then"
or the inner?

BCPL ~1965 had two constructs

    if p
       then test q
                 then ...
           ...
           else ...

"if-then" has no "else". If you want an "else" use "test-then-else".


AntC
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20180710/7d39d2e9/attachment.html>