I've read a few questions such as Scala vs Haskell discussing the merits of both languages or which to learn, but I already know that I'd like to learn Scala. I was a Java programmer at uni and now mainly use PHP.
I want to learn Scala as it looks like an improvement on Java for personal projects and I'd also like to learn a functional language to improve my knowledge as a programmer.
I am wondering if it would be a good idea to learn Haskell as an introduction to functional programming as it is purely functional so I'd properly learn it rather than haphazard using bits of functional in Scala without knowing why?
I'd also like to use Haskell for personal projects etc as it looks great, but I don't really see many real world applications of it, seems more used for academic stuff hence wanting to learn it to get the functional understanding then move on to Scala.
Speaking as someone who came from Java, and for whom Scala was a gateway drug to Haskell, I happen to think this is a great idea (learn Haskell first)!
Haskell is conceptually a much simpler language than Scala, and if your goal is to learn how to program functionally, you can't help but do so if you start with Haskell. By design, Scala supports a kind of "legacy mode" of coding in which you don't really have to change your Java (or PHP) habits too much if you don't want to. I think this is a strategic decision--and a good one!--meant to increase adoption amongst crusty Java stalwarts.
But that's not you! You're actually interested in learning something new... so why not go all-out? Learning functional programming in a pure setting, without the clutter and the temptation to regress into old habits, will crystallize the concepts in your brain.
Then by all means return to Scala and learn how it differs from Haskell; it is both weaker in some respects and stronger in others, but you will then be on a much better foundation to appreciate these differences.