teaching machines

CS 330 Lecture 25 – Currying and The One-Liner

April 5, 2013 by Chris Johnson. Filed under cs330, lectures, spring 2013.

Agenda

• what ?s
• what does this do?
• the Maybe/Option type
• currying and how to read Haskell function specifications
• find’ in an association list/hash
• point-free style
• foldl and foldr
• or’
• and’
• sum’
• product’
• min’
• foldr1′
• max’
• join’
• length’
• average’
• map’
• countOvers
• composing functions
• any’
• all’

TODO

• Play. And homework.

What Does This Do?

• char *mystrcpy(char *dst, const char *src) {
    for (; *src != '\0'; ++src, ++dst) {
      *dst = *src;
    }
  }
  
  int main(int argc, char **argv) {
    char s[1024] = "Bucephalus";
    mystrcpy(&s[1], s);
    printf("%s\n", s);
    return 0;
  }

• public static <T, U> List<U> foo(List<T> lot, Apply<T, U> f) {
    List<U> lou = new LinkedList<U>();
    for (T t : lot) {
      lou.add(f.func(t));
    }
    return lou;
  }

Code

today.hs

-- find' :: [(String, Integer)] -> String -> Maybe Integer 
find' [] needle = Nothing
find' (first:rest) needle
  | (fst first) == needle = Just (snd first)
  | otherwise = find' rest needle

ageBook = [("Sevy", 21), ("Chris", 32)]
getAge name =
  case find' ageBook name of Nothing -> "no such person"
                             Just age -> "this person is " ++ show age

cs330AgeBook = find' ageBook

-- getAge name = 
  -- case find' ageBook name of Nothing -> error "No such person" 
                             -- Just age -> age 

foldr' mix accum [] = accum
foldr' mix accum (first:rest) = mix first (foldr' mix accum rest)

-- and' list = foldr' (\item accum -> accum && item) True list 
-- or' list = foldr' (\item accum -> accum || item) False list 
and' = foldr' (&&) True
or' = foldr' (||) False
sum' = foldr' (+) 0
product' = foldr' (*) 1
reverse' = foldr' (:) []

Haiku