Allow user to enter DSL, rules or code?

I have a python library that allows me to define rules by chaining inputs and outputs of functions ( producer(grep(count( [print(),log() ] ))) This functions define a pipeline from left to right.

How could I allow users to define pipelines from a UI ?

I need to take a definition like for example "producer->grep->count->print,log" and interpret it so as to call the former code.

Do i need a parser? an AST ? What would be a good way to build the call sequence fron a representation of the code?