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'else' on new line not parsed at top level as part of preceding 'if'

4 messages · Mikael Jagan, Duncan Murdoch

Original
Mikael Jagan
# 
I wonder if there is room for improvement here:

     > str2lang("if (a) b1    else b0")
     if (a) b1 else b0
     > str2lang("if (a) b1 \n else b0")
     Error in str2lang("if (a) b1 \n else b0") : <text>:2:2: unexpected 'else'
     1: if (a) b1
     2:  else
         ^

It only occurs at top level.  When the parser knows that it needs to
"keep reading", e.g., because the call to 'if' is enclosed in braces,
it detects the 'else':

     > str2lang("{ if (a) b1    else b0 }")
     {
         if (a)
             b1
         else b0
     }
     > str2lang("{ if (a) b1 \n else b0 }")
     {
         if (a)
             b1
         else b0
     }

If people agree that the above is a deficiency/bug then I could create a
formal bug report.  Then others or I could start thinking about a patch
for the parser.

Apologies if this issue has been seen and discussed before (most likely
by people like me in the habit of dropping braces in calls to 'if' where
not strictly necessary).

Mikael
Duncan Murdoch
# 
This is well known, and it's necessary for reasonable behaviour in an 
interactive context.  Imagine typing the code you were parsing.  The 
first line is

  if (a) b1

At this point, the interpreter can't know that an else clause is coming. 
  Since the interpreter evaluates statements when they are complete, it 
will evaluate this and return b1 or NULL.

Putting the full expression in braces solves the issue, because this is 
clearly incomplete:

   { if (a) b1

Duncan Murdoch
On 2024-12-02 10:56 a.m., Mikael Jagan wrote:
Duncan Murdoch
# 
BTW, this is discussed in section 3.2.1 of "The R Language Definition".

Duncan Murdoch
On 2024-12-02 10:56 a.m., Mikael Jagan wrote:
Mikael Jagan
# 
Thanks - that makes sense.  It's also documented in help("if"), so it's
doubly clear now that I did not do my homework here.

Mikael
On 2024-12-02 11:09 am, Duncan Murdoch wrote: