I listed a number of classic papers on functional languages. I read these papers whenever I have spare time to refresh my mind.
[1] Why Functional Programming Matters, John Hughes, The University, Glasgow
[3] Monads for functional programming, Philip Wadler, University of Glasgow
[4] Theorems for free, Philip Wadler, University of Glasgow
I will keep updating the list of papers here. If you think I missed an important paper, please let me know.
Kwang Yul Seo 
You missed the most important one:
Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs
from John Backus, written in 1977. You can read more about it at Wikipedia:
http://en.wikipedia.org/wiki/John_Backus
http://en.wikipedia.org/wiki/Functional_programming#History
It’s easy to find on Google. Also you can download it here:
Click to access backus_turingaward_lecture.pdf
I haven’t read it yet…
Regards,
Albert Vila
Thanks! That should be the first classic paper in functional programming. I updated the list.
I would also suggest the classical “Out of Tar Pit” paper. It explores the process of creating large-scale systems with FP as a basis. I strongly recommend it!
Thanks! I will take a look at it.