"Tag Part in (...) is Protected" - wolfram-mathematica

Question

I am currently writing a module that's supposed to take some data points of 2-dimensional function (a 3 x N matrix) and draw contour plot of approximation based on those points (functions and variables for fitting are provided by user).
The "header" looks like this:
project4[dataPoints_, functionList_, fittingVarsList_, plotArgs___] :=
Module[{fitFunc, functionContourPlot, dataPointsXY, pointsPlot,
xList, yList},
Example of usage:
project4[data, {1, x, y, x y, x^2, y^2}, {x, y}]
(where data = {{x1,y1,f1}...})
After checking if the arguments are valid I do:
fitFunc = Fit[dataPoints, functionList, fittingVarsList];
To obtain the approximation.
Then I want to obtain plot of it by doing:
functionContourPlot = ContourPlot[fitFunc, {fittingVarsList[[1]], xMin, xMax},{fittingVarsList[[2]],yMin, yMax};
Which leads to an errors:
ContourPlot::write: Tag Part in {x,y}[[1]] is Protected. Show::gcomb:
"Could not combine the graphics objects in
Show[ContourPlot[fitFunc$2187,{{x,y}[[1]],xMin,xMax},{{x,y}[[2]],yMin,yMax}],"
What am I doing wrong?

Answer 1

The problem is ContourPlot having attribute HoldAll, which prevents Part evaluating.
Attributes#ContourPlot
You can fix it like this.
data = {{6, 4, 7.92}, {6, 5, 9.31}, {6, 6, 9.74},
{7, 4, 11.24}, {7, 5, 12.09}, {7, 6, 12.62},
{8, 4, 14.31}, {8, 5, 14.58}, {8, 6, 16.16}};
fittingVarsList = {x, y};
{xMin, xMax} = Through[{Min, Max}#data[[All, 1]]];
{yMin, yMax} = Through[{Min, Max}#data[[All, 2]]];
fitFunc = Fit[data, {1, x, y}, {x, y}]
This reproduces the problem :-
functionContourPlot = ContourPlot[fitFunc,
{fittingVarsList[[1]], xMin, xMax},
{fittingVarsList[[2]], yMin, yMax}];
The problem can be fixed by using With to create local variables :-
functionContourPlot =
With[{a = fittingVarsList[[1]], b = fittingVarsList[[2]]},
ContourPlot[fitFunc, {a, xMin, xMax}, {b, yMin, yMax}]]
If you remove HoldAll from the attributes of ContourPlot the first version works ...
Unprotect#ContourPlot;
ClearAttributes[ContourPlot, HoldAll]
... but that would be reckless programming.