If you refer to nextflow,the syntax is basically groovy
Edit: functional as in, it's not half-hearted, not functional as in functional programming.
The Nextflow scripting language is an extension of the Groovy programming language. Groovy is a powerful programming language for the Java virtual machine. The Nextflow syntax has been specialized to ease the writing of computational pipelines in a declarative manner.
https://www.nextflow.io/docs/latest/script.html?highlight=gr...
http://docs.groovy-lang.org/docs/latest/html/documentation/c...