Re: user-defined function & string file name

[<br...@ph...>]

Ethan A Merritt wrote:

> The difficult case (which this is not an example of) is
> distinguishing a user-defined function that returns the name of a
> file from a user- defined function that returns a numerical value.

We may be closer to that case than you realize.

     plot fdat

could be a plot of a file (if fdat is a string-valued variable), a
linear function (if 'set dummy fdat' is in effect), or a constant (if
fdat is a numerical variable). There are four cases, distinguished by
the attribute pairs string<-->numeric and literal<-->expression.

There is not actually a distinction between function and expression to 
be made here.  I.e. the 'plot' command line parser is not trying to find 
a function --- it's trying to find an expression that it can evaluate, 
and a simple variable or even a literal fit that bill nicely. That's why 
all of

    plot 5
    plot a
    plot x
    plot 5*a+f(x)

work.  To recognize that a file name is being input, the parser needs 
some way of figuring out that what the type of a given expression is. 
Before string variables, that was simple: strings could only be 
literals, and those were easy to recognize by the opening ' or ".  Now 
it's trickier, and in the example discussed here, it fails.

In the case of a 'plot fdat', there's no syntactic hint at all --- you 
have to actually check the type of fdat to see if this is a filename or 
a numeric variable.  I.e. the exact same plot command line could produce 
a function or a data plot, at different times in the same gnuplot session.