Re: Awk output redirection to expression - defined or not?

[Manuel Collado]

El 25/5/23 a las 15:37, Ed Morton escribió:
> [...]
> > Historical practice has been that:
> > getline < "a" "b"
> >
> > is parsed as:
> >
> > ( getline < "a" ) "b"
> >
> > although many would argue that the intent was that the file *ab* 
> > should be read. However:
> >
> > getline < "x" + 1
> >
> > parses as:
> >
> > getline < ( "x" + 1 )
> >
> > ...
> >
> > Since in most cases such constructs are not (or at least should not) 
> > be used (because they have a natural ambiguity for which there is no 
> > conventional parsing), the meaning of these constructs has been made 
> > explicitly unspecified.
>
> and:
>
> > The *getline* operator can form ambiguous constructs when there are 
> > unparenthesized binary operators (including concatenate) to the right 
> > of the '<' (up to the end of the expression containing the *getline*). 
> > The result of evaluating such a construct is unspecified
>
> but nothing about output redirection. I know gawk doesn't require parens 
> around the expression for output redirection but other awks do (e.g. see 
> https://stackoverflow.com/q/21093626/1745001) and it's not obvious to me 
> why `getline < "a" "b"` should be undefined behavior while `print > "a" 
> "b"` wouldn't be so intuitively if one of them is undefined then so 
> should the other be.

Well, there is a clear difference. 'getline' is an expression term, 
while 'print' is a statement. IMHO, in the first case the '<' appears as 
an operator, parsed according to operator precedence ('<' after getline 
has higher precedence than '<' as comparison operator). In the second 
case '>' is parsed as a keyword.

HTH.
--
Manuel Collado - http://mcollado.z15.es