syntax - How do I identify the source of a "Tag times protected" error?

I have some code that looks like

Table[
    a = 1;
    b = {2, 3}
    c = i;,
    {i, 2}
]

which gives an error:

Set::write: Tag Times in c {2,3} is Protected. >>

In this case, it is quite clear that there is a semicolon missing after b = {2, 3}, which is causing this error. However, sometimes I encounter this in large code blocks spanning several lines, which is very difficult to debug.

How can I automate this semicolon hunting to make debugging easier?

Answer

Here is a function findBadSets that will find any explicitly bad Set/SetDelayed attempts in a given expression. Simply wrap it around a syntactically complete block of code, or follow the block with // findBadSets and the errors are printed one per row, protected symbol followed by complete left-hand side for each bad Set:

(* your example *) // findBadSets

Code for the function:

SetAttributes[findBadSets, HoldFirst]

findBadSets[expr_] :=
  Cases[
    Unevaluated @ expr,
    (Set | SetDelayed)[bad : head_Symbol[___], _]
      /; MemberQ[Attributes@head, Protected] :>
        HoldForm[Row[{head, bad}, Spacer[50]]],

   -1] // Column

See also: