Have you read your CTCS today?

Category Theory for Computing Science is a fundamental book for any person willing to be a language designer, and it's free:
http://www.math.mcgill.ca/triples/Barr-Wells-ctcs.pdf

>>6-8
I never claimed the rule to be general. SBCL and SICP are both free yet they are priceless in value. Most proprietary software has a non-negligible price tag attached to it, yet is often worthless. This paragraph disproves both directions on the `general statement' double implication.

CTCS and Haskell, on the other hand, are both free and worthless, which is why the colloquialism ``you got what you paid for'' applies.