Programming Languages (CSC-302 98S)

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# Outline of Class 27: Lazy Functional Programming in Haskell

Held: Wednesday, April 8, 1998

• Any question on assignment five?
• Reminder: there's a cool talk during the math journal club tomorrow.
• Today we'll be moving from Scheme to Haskell so as to explore other issues in functional programming.
• I'll admit that I do most of my Haskell programming on the Mac, so I may rely on the exotic programming languages folks for help running Haskell on our HPs.
• We had a short assignment for today: what is the value of the following Scheme expression and why?
```(let ((a 2))
(+ a
(let ((a 4)) a)
a))
```

• Similarly, what's the value of the second of these two expressions and why?
```> (define eh (let ((a 5)) (lambda () a)))
> (let ((a 2)) (eh))
```

• Haskell is a functional language that provides a somewhat different perspective on functional programming than Scheme.
• Haskell is typed (with an automatic type inference system).
• Haskell is lazy rather than eager (it delays evaluation of expressions "as long as possible"). We'll return to this concept soon.
• Haskell is pure: functions don't have side effects. We also say that Haskell functions are referentially transparent: the value they return depends only on the arguments they are sent.
• All functions in Haskell are curried.
• Haskell has a different syntax (whoopdedoo)
• The core Haskell web page is at `http://haskell.org/` . The Gentle Introduction to Haskell at `http://haskell.org/tutorial/index.html` is a good place to start learning the stuff that I don't teach you.

### Syntax

• Like many functional languages, Haskell has a relatively simple syntax.
• Function application is notated by the function, a space, and its argument. For example, `f a` represents "apply f to a".
• No parentheses are necessary (except for clarity or to circumvent precedence rules).
• Since Haskell is Curried, the apply operation (a space) is left-associative. For example, `f a b` is "apply f to a; apply the resulting function to b".
• To get the alternate interpretation (apply a to b, then apply f to the result), one would write `f (a b)`.
• Mathematical operations can be written in infix notation, as in `a + b`.
• Haskell uses capitalization to indicate the roles of various program components. In Haskell, these capitalization standards are requirements rather than recommendations.
• Functions, variables, and type variables begin with a lowercase letter.
• Types and type constructor begin with an uppercase letter.
• Data types are described with the `data` keyword, using the form
```data Typename = Definition
```

• Functions are described by giving the function name, any arguments, an equals sign, and the body. For example,
```fact n = if (n == 0) then 1 else n * (fact (n - 1)
```

• You can also describe the types of things (e.g., functions), two colons, and a type expression. For example,
```fact :: Int -> Int
```

• List are written with square brackets and commas between the elements. All the elements must be the same type. For example,
```[4, 2, 4, 1]
```

• Vectors (also records and products) are written with parentheses and commas between the elemnts.

Back to Scheme, Concluded
Outlines: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
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