Sure you can, if you have an editor and REPL worth their salt. Select af(param)and evaluate it. There's what you're calling "a". Select bf(af(param) and evaluate it. There's what you're calling "b". How one carries out "select and evaluate" varies depending on the language, REPL, and editor in use, but there's almost always (always, in my experience) an easy way to do it.
Bonus: if it's a pure functional language (no side-effects) you can do it as many times as you like without messing anything up, and you didn't have to invent any otherwise-useless variables.
Sure you can, if you have an editor and REPL worth their salt. Select af(param)and evaluate it. There's what you're calling "a". Select bf(af(param) and evaluate it. There's what you're calling "b". How one carries out "select and evaluate" varies depending on the language, REPL, and editor in use, but there's almost always (always, in my experience) an easy way to do it.
Bonus: if it's a pure functional language (no side-effects) you can do it as many times as you like without messing anything up, and you didn't have to invent any otherwise-useless variables.