[alex_lav]

I would love to be educated. I've seen claims about the merit and value of functional programming throughout my (nowadays relatively long) programming career. In practice I've never once seen those values or merits come to fruition - just the same cycle all software goes through.

My very direct experience recently has been Scala + cats resulted in the same buggy nonperformant software it was meant to prevent. I understand that bad programmers produce bad programs, regardless of language, but I feel pretty strongly that good tools prevent some amount of the typical "bad" that makes bad programs bad (ignoring the obviously maliciously bad examples). So I don't really understand, and would like to understand, if, how and when pure FP (and I suppose FP in general) actually improve quality of code/life outside of toy examples.

[mrkeen]

The bottom line is that pure FP means that the same input to a function gives you the same output.

When you debug, you just give the program the same input which was problematic and you get to reproduce the error.

Persistent data structures make it less wildly inefficient to do so.