Is there a way to convert -- or evaluate -- a string to a pitch?

[Kevin Cole]

Suppose I have two environment variables FROM_KEY and TO_KEY

and in my lilypond source I have:

%%%%%%%%%%%%%%%%
fromkey = #(if (getenv "FROM_KEY") (getenv "FROM_KEY") "c")
tokey   = #(if (getenv "TO_KEY")   (getenv "TO_KEY")   fromkey)
...
\transpose \fromkey \tokey { \melody }
%%%%%%%%%%%%%%%%

I get:

error: wrong type for argument 1.  Expecting pitch, found "c"
error: wrong type for argument 2.  Expecting pitch, found "c"

What am I missing in order to evaluate the string as a pitch?