Chapter 2: Starting Out

Learn You a Haskell for Great Good!

Ready, set, go!

  • use parentheses for negative number.
    • 5 * (-3) instead of 5 * -3.
  • binary operation on same type
    • True == 5 is not allow because comparing bool with number
    • 5 + “llama”is not allow because comparing number with string
    • 5 + 4.0 is ok because the 5 adapted to become a floating-point number

Baby’s first functions

  • Infix examples are +, -, *, /, ==, …etc.
  • prefix examples are succ, min, … etc.
  • function application has highest precedence
  • div 92 10 is equivalent to 92 ‘div’ 10
  • to load a script (without .hs)
    • :l [filename]
  • function in Haskell don’t have to be in any particular order
  • if then else statement must have an else statement
  • ‘ is a valid character for function
  • functions can’t begin with uppercase letters
  • function without parameter is a definition

An intro to lists

  • homogenous (list of elements of the same type)
  • let keyword define a name
  • lists are denoted by square brackets []
  • a string is just a list of characters (“string”, character – ‘c’)
  • combine two lists
    • list ++list
  • the con operator put an element (not a list) at the beginning of the list
    • element : list
  • [] is an empty list
  • get the x element out of the list where indices start at 0.
    • list !! x
  • list can also contain list
  • return the first element of the list, (e.g. head [5,4,3,2,1] gives 5)
    • head list
  • return the same list but without the head, (e.g. tail [5,4,3,2,1] gives [4,3,2,1])
    • tail list
  • return the last element of the list, (e.g. last [5,4,3,2,1] gives 1)
    • last list
  • return the same list but without the tail (e.g. init [5,4,3,2,1] gives [5,4,3,2])
    • init list
  • return the length of the list
    • length list
  • check if the list is empty
    • null list
  • return the same list in reverse order
    • reverse list
  • return the same list using only x number of the elements from the beginning
    • take x list
  • return the same list without the x number of the elements from the beginning
    • drop x list
  • return the sum of a list of numbers
    • sum list
  • return the product of a list of numbers
    • product list
  • check whether a is in the list
    • elem a  list

Texas ranges

  • to make a list from a to b
    • [a .. b]
  • to make a list from a to b with step s (e.g. [3,6..20] gives [3,6,9,12,15,18])
    • [a,(a+s)..b]
  • to make a infinite list with step s
    • [a,(a+s)..]
  • takes a list and cycles it into an infinite list
    • cycle list
  • takes an element and produce an infinite list of just that element
    • repeat x
  • give a list of x elements of a
    • replicate x a

I’m a list comprehension

  • a list comprehension that constraint the variable from the criteria before calculating the function of the element into a list. (filtering)
    • [function | predicate 1, predicate 2 ..]
  • check whether x is odd
    • odd x
  • _ means we don’t care what we’ll draw from the list

Tuples

  • can contain a combination of several types
  • use parentheses () to denoted a tuple
  • list of tuple will force the list to use the same tuple type
  • takes a pair and returns its first component
    • fst tuple
  • takes a pair and return its second component
    • snd tuple
  • combine corresponding elements of both list and put them in a tuple and return a list with the same length as the shorter list
    • zip list1 list2
  • Example: right triangle
    • find the dimension of a right triangle with perimeter equal to 24.
    • let rightTriangles = [ (a,b,c) | c <-[1..10], b <-[1..c], a <-[1..b], a^2+b^2==c^2, a+b+c==24]

Other commands

  • quit ghci
    • :quit
  • to set default text editor
    • :set editor [editor]
  • to edit a file
    • :edit [filename.hs]
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