Download Foundations of Programming Languages * Course Overview

Main Points of Haskell
• Functional programming
• Laziness
• Purity (i.e. no side effects)
Main Points of Haskell
• Functional programming
– No states. Just values.
– No commands. Just math functions and
declarations.
x=1
• Imperative: assignment
• Functional: declaration
Main Points of Haskell
• Functional programming
if … then … else
– Imperative:
if <Bool> then <Commands> else <Commands>
– Functional:
if <Bool> then <Value> else <Value>
“if” can be viewed as a function: if :: Bool -> a -> a -> a
if-then-else is just a syntactic sugar.
Main Points of Haskell
• Functional programming
– “Pure” math functions. Not procedures.
– Transform inputs to outputs. No side effects.
Main Points of Haskell
• Functional programming
– No loops. Use recursion instead. Why?
– Recursion: define sth using itself
– To solve a problem on recursively-defined data
(e.g. a list, a tree), think of a recursive solution
Main Points of Haskell
• Functional programming
– No states. Just values.
– No commands. Just “pure” math functions and
declarations. Not procedures.
• Laziness (i.e. call-by-need)
• Purity (i.e. no side effects)
Main Points of Haskell
• Laziness:
– Evaluation of function arguments is delayed as
long as possible
fxy=x+2
f 5 (22^3579): 22^3579 will not be evaluated
When to evaluate an argument expression?
Main Points of Haskell
• Laziness:
– Argument expressions are only evaluated when
pattern-matched
data Maybe = Nothing | Just Int
f1 :: Maybe -> [Maybe]
f1 m = [m, m]
f2 :: Maybe -> [Int]
f2 Nothing = []
f2 (Just x) = [x]
Main Points of Haskell
• Laziness:
– Argument expressions are only evaluated when
pattern-matched
– Only as far as necessary for the match to proceed,
and no farther!
f2 Nothing = []
f2 (Just x) = [x]
f2 (safeHead [3^500, 2]): f2 will force the evaluation of the
call to (safeHead [3^500, 2]), which’ll evaluate to (Just 3^500)
Main Points of Haskell
• Purity (no side effects)
– Side effects: anything that causes evaluation of an
expression to interact with something outside
itself.
The issue: those effects are time-sensitive. E.g.
• Modifying a global variable
• Printing to the screen
• Reading from a file
Main Points of Haskell
• Laziness forces purity
– Lazy evaluation makes it hard to reason about
when things will be evaluated
– But sides effects are time-sensitive
– Hence including side effects in a lazy language
would be extremely unintuitive
f (print(“hello”), print(“bye”))
Main Points of Haskell
• Functional programming
• Laziness
• Purity (i.e. no side effects)