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Introduction to Expressions and Equations

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


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) -- 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 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 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: :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
-- Evaluate an expression --
Prelude> 3+4

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

© University of Glasgow
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