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Introduction to Expressions and Equations

Basic expressions and equations in Haskell
The Zen of Haskell
© University of Glasgow

Expressions

Haskell has no statements, only expressions!

  • In an imperative language like C or Java,
    • there are expressions that denote small scale computations (2*x), and
    • statements that handle sequencing, looping, conditionals, and all the large scale operation of the program.
  • Pure functional programming languages don’t have any statements — no assignments, no jumps.
  • Instead, all computation is performed by evaluating expressions
  • So, let’s start with expressions!
    • (We’ll still be working on expressions at the end of the course, since that’s all there is.)

Examples of integer expressions

An expression evaluates to a result (usually written (e rightsquigarrow r) but we’ll use e -- > r). Haskell uses a similar notation for numbers and operators as most languages:

 2 -- > 2
3+4 -- > 7
3+4*5 {equivalent to 3+(4*5)} -- > 23
(3+4)*5 {equivalent to 7*5} -- > 35

Syntax of expressions

  • Parentheses are used for grouping, just as in mathematics.
  • If you don’t need parentheses for grouping, they are optional.
  • Operators have precedence, e.g. ( * ) has “tighter” precedence than ( + ), so (2 + 3 * 4) means (2 + (3 * 4)).
  • Use the reference documentation for complete list of operators and their precedences, if you need them.

Function applications

  • Expressions can contain function calls.
  • A function takes argument(s), performs some computation, and produces result(s).
  • The function abs gives the absolute value of a number.
  • To use a function, you apply it to an argument. Write the function followed by the argument, separated by a space.
 abs 5 -- > 5
abs (-6) -- > 6

Parentheses are for grouping

Good style

 2+3*5
2+(3*5) -- might be clearer to some readers
abs 7

You don’t need parentheses. The following are legal, but they look silly:

 (2) + ((3+(((((5)))))))
abs (5)
abs (((5)))

Functions with several arguments

  • min and max are functions that take two arguments.
  • The arguments are given after the function, separated by whitespace.
  • Write min 3 8, don’t write min(3, 8);
 min 3 8 -- > 3

max 3 8 -- > 8

Precedence of function application

  • Function application binds tighter than anything else.
  • So f x + 3 means (f x) + 3 and not f (x+3)
  • If an argument to a function is an expression, you’ll need to put it in parentheses.

Equations

Equations give names to values

  • Equations are used to give names to values.
answer = 42
  • An equation in Haskell is a mathematical equation: it says that the left hand side and the right hand side denote the same value.
  • The left hand side should be a name that you’re giving a value to.
  • Correct: x = 5*y
  • Incorrect: 2 * x = (3*x)**2 – Reassignment is not allowed in a pure FPL

Equations are not assignments

  • A name can be given only one value.
  • Names are often called “variables”, but they do not vary.
  • In Haskell variables are constant!
 n = 1 -- just fine!
x = 3*n -- fine
n = x -- Wrong: can have only one definition of n
  • Once you give a value to a name, you can never change it!
  • This is part of the meaning of “pure” and “no side effects”

What about n = n+1?

  • In imperative languages, we frequently say n := n + 1
  • This is an assignment, not an equation!
  • It means (1) compute the right hand side, using the old value of n; then (2) discard the old value of n and overwrite it with the new value.
  • There are no equations in imperative languages like C and Java.

  • In Haskell, it is valid to write n = n + 1.
    • This is an equation, not an assignment!
  • It means: compute the value of n that has the property that n = n + 1.
  • Haskell will try, and it will fail.

How can you compute without assignments?

  • Think of an assignment statement as doing three things:
    1. It evaluates the right hand side: computing a useful value.
    2. It discards the value of the variable on the left hand side: destroying a value that might or might not be useful.
    3. It saves the useful value of the RHS into the variable.
  • In a pure functional language
    • We never destroy old values.
  • We just compute new useful ones.
  • If the old value was truly useless, the garbage collector will reclaim its storage.

Try Haskell!

Haskell in your browser

Installing Haskell

  • You can install the Haskell compiler/interpreter on your own computer. Go to https://www.haskell.org/platform to get the Haskell Platform for your system, it is very easy to install. For more details see Installing Haskell for Yourself in Week 2.
  • All software used in this course is free software.
  • Try experimenting with the expressions shown in this lecture.
  • And try some experiments of your own.

The Haskell interpreter ghci

To launch the interactive Haskell interpreter ghci, just type ghci in your terminal:

[wim@fp4 ~]$ghci
GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude>
-- Evaluate an expression --
Prelude> 3+4
7

To exit the Haskell interpreter, type :quit at the interactive prompt.

© University of Glasgow
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