[R] How to use a string to refer a function?

[blue sky]

> f=function(){ print('in f')}
> attr(f, 'source')
[1] "function(){ print('in f')}"

I have the above simple function. I can use the variable f to refer to
the function and get the function source.

Suppose that I have 'f' as a string (say I get it from ls()), could
somebody let me know how to get the function from the name 'f'?