Munich Clojure Workshop - 07 - Let Over Lambda

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Transcript

1. Let over Lambda Friday, October 11, 13
2. What is a lambda? Friday, October 11, 13
3. A lambda is a function Friday, October 11, 13
4. A lambda is a function (fn [x] (* x 2)) Friday, October 11, 13
5. But why the hell"lambda"? Scheme calls it that. Also: lambda calculus (lambda (x) (* x 2)) Friday, October 11, 13
6. What are scopes? Friday, October 11, 13
7. What are scopes? a is “in scope”, b maybe. The bindings(assignments) a function can “see”: (fn [a] b) Friday, October 11, 13
8. (fn [a] (fn [b] (+ a b))) Takes a and returns a function. A FunctionFactory if you please! The returned function takes b and returns a + b as result. Friday, October 11, 13
9. This can be useful if you want to preconfigure a function: (defn addition [a] (fn [b] (+ a b))) (def increment (addition 1)) (map increment [1 2 3 4 5]) ;=> (2 3 4 5 6) Friday, October 11, 13
10. Why should I want this? • Ever wrote a callback function? They often expect fixed arities. • Or just a higher order function like map? Friday, October 11, 13
11. A hypothetical example GUI API (let [address (get-postal-address-of :santa)] (button :text "Send to santa" :onclick (fn [event widget] (send-mail-to address))])) 1 2 3 4 5 6 Function in line 5 has access to address, but only takes event and widget as arguments. Friday, October 11, 13
12. What is this exactly? This returns a function, a so-called “closure”. It “closes over” a scope, so it knows that a is 1. (addition 1) Friday, October 11, 13
13. Closure Friday, October 11, 13
14. Clojure Friday, October 11, 13
15. I see what you did there, Rich Hickey. Friday, October 11, 13
16. ; old and busted ;-) (def increment #(+ 1 %)) ; new hotness (def increment (partial + 1)) (map increment [1 2 3 4 5]) ;=> (2 3 4 5 6) It takes a function(+) and some arguments(1) and returns a closure with the arguments pre-set. Preconfigure a function which does not return a closure? partial to the rescue! Friday, October 11, 13
17. Other fun things with lambdas and scopes Friday, October 11, 13
18. Dynamic scoping with binding Friday, October 11, 13
19. Data access control Friday, October 11, 13
20. Data access control As we'll implement in the exercises. Friday, October 11, 13