Hacker News Reader

Comment by reirob | Hacker News Reader

On the related Reddit thread [0] a commenter links to a very nice blog that evaluates Frege [1] in a very succinct way. To answer your question by quoting this blog post:

"Wrapping mutable Java stuff in Frege means that everything will end up in an ST monad, which is almost always IO. That means that using Java’s HashSet will force anything that uses that to be in the IO monad itself. So there goes purity and referential transparency, which are some of the best parts of Haskell in the first place."

[0] https://www.reddit.com/r/haskell/comments/3gr7y6/infoq_frege...

[1] http://taylor.fausak.me/2015/06/25/frege-a-jvm-haskell/

EDIT: formatting

View on HN

Thanks for this.

This is a bit misleading though I think, as you can actually unwrap and ST unlike an IO - my most common use for ST is actually to use a mutable object in a case where it's inefficient to keep copying immutable (e.g.: large vector or matrix manipulations) - then when you're done the ST monad can be unwrapped and you get the final structure - the entire function remains pure but inside of it, there are calculations that rely on state - but no state enters or leaves the function per say. This would mean using a hash set would be out of the question _for passing around outside of an ST monad_ but you could actually use it internally within functions.

> This would mean using a hash set would be out of the question _for passing around outside of an ST monad_ but you could actually use it internally within functions.

Yes, but this is what the author was saying: you end up writing your code in ST. Remeber, there is no (general) way to freeze a mutable collection that was passed and then use it as if it was immutable. There can be a dozen different threads be busy modifying it.

However, as immutable objects are becoming more mainstream even in Java, there will still plenty of opportunities to make polyglot JVM programs employing pure code. Just not with the JAVA collection classes.

Reply on news.ycombinator.com