Detecting DownValue/OwnValue conflicts

Occasionally I run into the following scenario -- one procedure uses a global function f[x] to do some computation, meanwhile another procedure does f=5 which turns all subsequent calls f[x] into 5[x]

I know about localizing variables, but I prefer to use global variables early in the development.

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I can't think of a legitimate reason to mix OwnValues and DownValues. Is there a way to implement a warning message when situation like above happens?

In addition to tips below, default context can be found in OptionsInspector under Cell Options/Evaluation Options/CellContext

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You can use Protect[f] to avoid further assignments (of any kind) and Unprotect[f] to allow them again.

You could also do some simple hackery with up-values to prevent Set[f, ...] from actually putting own-values on f but still allowing the assignment of down-values without having to use Protect/Unprotect:

In[76]:= ClearAll[f]

In[77]:= f /: Set[f, x_] := x

In[78]:= f = 7

Out[78]= 7

In[79]:= f

Out[79]= f

In[80]:= f[x_] := x + 1

In[81]:= f[1]

Out[81]= 2

The above transparently blocks the use of Set on f. Issuing an actual message is straightforward too:

In[93]:= f::noov = "Blocked attempt to assign `1` as an OwnValue of f";

In[94]:= f /: Set[f, x_] := (Message[f::noov, x]; x)

In[95]:= f = 7

During evaluation of In[95]:= f::noov: Blocked attempt to assign 7 as
   an OwnValue of f

Out[95]= 7

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This doesn't answer the original question about generating a warning message, but Formal Symbols are handy for use as global symbols. They are automatically protected and therefore cannot be accidentally redefined. You enter a formal symbol using the keystrokes <ESC>$f</ESC>, where f can be any letter. The disadvantage is that it takes four keystrokes to enter the symbol instead of one.