Thanks Oliver. That's good enough.
I was ever curious about whether parse String to the function rather
than a mapping.
GHC has the ability to embed an interpreter. You do not want to use it. If you want Perl/Python/Ruby, please use those; they are interpreters and have built-in embedded interpreters for use by programs.
(Note that C and C++ are not generally interpreted and also don't have embedded interpreters either.)