It's a deliberate mistake, as is "Learn You".
I think this is based on a comedy routine by Baron Sacha Cohen called Borat.
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I'm inclined to think it's a parody of "For great justice!"
from the badly-translated game, Zero Wing. http://en.wikipedia.org/wiki/All_your_base_are_belong_to_us
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Haskell is a very mathematical programming language.
It is typically learned by mathematically inclined people who have big brains.
That makes the subject matter frightening to many people.
It triggers their "math anxiety".
The title is deliberately silly so as not to make people feel anxious.
Haskell books are typically pedantic and formal.
The informal, grammatically incorrect, silly style of the title is kind of promise to the reader that the book won't be overly pedantic and formal.
I think also that the author, who is Slovenian, may be making fun of his own English skills.
(The English inside the book however is very good.)
In better English, the title might be something like, "Learn Haskell for great benefit".
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I think it's a Haskell joke, playing on the functional programming style used in that language.
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According to this post the phrasing originated in the title of the Haskell tutorial " Write Yourself a Scheme in 48 hours ", written in 2006, and was simply a (not that ungrammatical, but certainly awkward) "portmanteau of two different common memes at the time": the "Teach yourself X in 21 days books", and Peter Norvig's semi-parody "Teach yourself programming in 10 years".
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I'd be more inclined to interpret "for Great Good" as a form of "for the Greater Good".
In that case it corresponds with "learning Haskell is good for whole world" per your suggestion.
Compare to "Making the world a better place, one person at a time".
english.stackexchange.com/users/5670/Vincent Vancalbergh
There is a site  learn you a haskell  with the title "Learn You a Haskell for Great Good!". Does "Great Good" mean "very very good"? Does the whole phrase mean "learning Haskell is good for you" or "learning Haskell is good for the whole world" or something else?
What does "great good" mean in the tutorial title "Learn You a Haskell for Great Good!"?
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<meaning>
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There are a number of possible meanings for the title "Learn You a Haskell for Great Good!". The title could be deliberately silly so as to put people at ease or the author, being Slovenian, may be poking fun at his own English skills. Furthermore, it may be a joke about the programming style used in the Haskell language. In any case, a better re-phrasing of the title would probably be "Learn Haskell for great benefit". It's a joke based on a deliberate grammatical error.
There are a number of possible meanings for the title "Learn You a Haskell for Great Good!". The title could be deliberately silly so as to put people at ease or the author, being Slovenian, may be poking fun at his own English skills. Furthermore, it may be a joke about the programming style used in the Haskell language. In any case, a better re-phrasing of the title would probably be "Learn Haskell for great benefit". "For Great Good"  could be interpreted as a form of "for the Greater Good".  On the other hand it could be said that the title is deliberately silly and with mistakes so as not to make people feel anxious. Haskell is a mathematical programming language and Haskell books are typically pedantic and formal. The informal, grammatically incorrect, silly style of the title is kind of promise to the reader that the book won't be overly pedantic and formal.  
In better English, the title might be something like, "Learn Haskell for great benefit".  However, "for Great Good" may be interpreted as "for the Greater Good". There is no agreement on what "great good" means in this statement, but it could mean "great justice", "great benefit" or "the greater good".
It's a joke based on a deliberate grammatical error with the aim of making Haskell look less threatening. It's a joke based on the style of the Haskell language. It's a joke based on the style of the teach yourself book.
The title is deliberately silly and with mistakes so as not to make people feel anxious. Haskell is a mathematical programming language and Haskell books are typically pedantic and formal. The informal, grammatically incorrect, silly style of the title is kind of promise to the reader that the book won't be overly pedantic and formal.   "For Great Good"  could be interpreted as a form of "for the Greater Good".  
There is no agreement on what "great good" means in this statement, but it could mean "great justice", "great benefit" or "the greater good".