I want a function that works like in the following example: it gets an expression which contains a variable var and evaluate it setting setting var = value. I used Block to avoid expr being evaluated if variable var holds a value in the notebook. It seems that my function works correctly - I made alo a trace - but I get var inside block coloured red and I think it's a warning from Mathematica. Can you explain to me why I obtain it? Is my code wrong? Is there a better way to obtain the desired behavior?

SetAttributes[myFunc, HoldFirst];

myFunc[{expr_, var_}, value_] :=
  Block[{var}, expr /. var :> value]

Thanks to all, Andrea

I want a function that works like in the following example: it gets an expression which contains a variable var and evaluate it setting setting var = value. 

I used Block to avoid expr being evaluated if variable var holds a value in the notebook. It seems that my function works correctly - I made also a trace - but I get var inside block coloured red and I think it's a warning from Mathematica. Can you explain to me why I obtain it? Is my code wrong? Is there a better way to obtain the desired behavior?

SetAttributes[myFunc, HoldFirst];

myFunc[{expr_, var_}, value_] :=
  Block[{var}, expr /. var :> value]

I want a function that works like in the following example: it gets an expression which contains a variable var and evaluate it setting setting var = value. I used Block to avoid expr being evaluated if variable var holds a value in the notebook. It seems that my function works correctly - I made alo a trace - but I get var inside block coloured red and I think it's a warning from Mathematica. Can you explain to me why I obtain it? Is my code wrong? Is there a better way to obtain the desired behavior?

SetAttributes[myFunc, HoldFirst;
 myFunc[{expr_, var_}, value_] :=
  Block[{var}, expr /. var :> value]

Thanks to all, Andrea

I want a function that works like in the following example: it gets an expression which contains a variable var and evaluate it setting setting var = value. I used Block to avoid expr being evaluated if variable var holds a value in the notebook. It seems that my function works correctly - I made alo a trace - but I get var inside block coloured red and I think it's a warning from Mathematica. Can you explain to me why I obtain it? Is my code wrong? Is there a better way to obtain the desired behavior?

SetAttributes[myFunc, HoldFirst];
 
myFunc[{expr_, var_}, value_] :=
  Block[{var}, expr /. var :> value]

Thanks to all, Andrea