Error handling in R is odd

So you want to run some code that may throw an error? This is somewhat less common with R than with e.g. php.

It is quite simple in Python:

The try statement works as follows.

  • First, the try clause (the statement(s) between the try and except keywords) is executed.
  • If no exception occurs, the except clause is skipped and execution of the try statement is finished.
  • If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the except keyword, the except clause is executed, and then execution continues after the try statement.
  • If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.

Let’s try something simple in iPython:

In [2]: try:
   ...:     "string"/7
   ...: except:
   ...:     print("Can't divide a string")
   ...:
Can't divide a string

Simple stuff.

Now, let’s do the same in R:

tryCatch("string"/7,
         error = function(e) {
           print("Can't divide a string")
         }
)

Notice how I had to add an anonymous function? Well apparently this is how it has to be done in R. The parameter to the function, e, is not even used. It would be better if one could simply do this:

tryCatch("string"/7,
         error = {
           print("Can't divide a string")
         }
)

But no, then you get:

[1] "Can't divide a string"
Error in tryCatchOne(expr, names, parentenv, handlers[[1L]]) : 
  attempt to apply non-function
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