https://tutorial.ponylang.io/expressions/object-literals.html?h=#lambdas
where it says
env.out.print(lambda("hello world"))
 
I note that this does not compile. I checked, and lambda is not itself a keyword; it is not listed here https://tutorial.ponylang.io/appendices/keywords.html
As a beginner, this is very confusing.
I think it might be the result of a simple bug in the compilation of the code snippet that generates it.
I'm not sure what it should say, so I cannot suggest a fix off hand.