formula argument evaluation

I have a simple function such as:

foo <- function(x) {
     call <- lapply(match.call(), deparse)
     testit <- capture.output(tryCatch(eval(x), error = function(e) e))
     if (grepl("Error", testit)) {
         return(call$x)
     }
}

and I would like to detect a formula when x is not an object:

# this works

foo(A + B)

[1] "A + B"

# but this doesn't

foo(A + B => C)

Error: unexpected '=' in "foo(A + B ="

Can I prevent it from evaluating the "=" sign?
The addition sign "+" hasn't been evaluated, and I was hoping the "=" would
not get evaluated either. The "=>" sign is important for other purposes,
not related to this example.

Thank you in advance,
Adrian

--
Adrian Dusa
University of Bucharest
Romanian Social Data Archive
Soseaua Panduri nr.90
050663 Bucharest sector 5
Romania

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