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Expressions

  • Programs are structured into modules (files)
  • Modules contain declarations (statements).
  • The most important form of declarations are bindings for new constants and functions.
  • In such bindings, expressions play the central role.

Therefore we are going to look at expressions before everything else.

A Haskell expression is a (possibly nested) terms built up from constants and function calls.

  • An important property of expressions is that they can be evaluated, yielding a value.
  • Values are themselves expressions that cannot be evaluated any further.
  • You can type expressions into GHCi. GHCi will then try to evaluate the expression and print its resulting value

Examples of values​

GHCi> 2
2

GHCi> 'x'
'x'

GHCi> "Haskell"
"Haskell"

GHCi> True
True

GHCi> [1,2,4]
[1,2,4]

Examples of function calls​

GHCi> not True
False

GHCi> min 7 2
2

GHCi> 2 + 3
5

GHCi> 3 : [10,99]
[3,10,99]

GHCi> take 2 [1,3,9,27,81]
[1,3]

GHCi> map odd [1,2,3,4,5]
[True,False,True,False,True]

Operators are functions​

Only syntactic differences between symbolic and alphanumeric function names.

Symbolic identifiers (operators) are infix by default, and can be made prefix by enclosing them in parentheses.

GHCi> 6 + 9
15

GHCi> (+) 6 9
15

Alphanumeric identifiers are prefix by default, and can be made infix by enclosing them in backquotes.

GHCi> max 12 20
20

GHCi> 12 `max` 20
20

Space is function application

min 7 2 -- function applied to two arguments

Parentheses are used for grouping

GHCi> min 7 (2 + 6)
7

GHCi> min 7 2 + 6
8

Function application binds stronger than operators.

GHCi> reverse (reverse [1,2,3])
[1,2,3]

GHCi> sum (filter odd [1,2,3,4,5])
9

GHCi> take 1 "Haskell" ++ drop 4 "Haskell"
"Hell"

Anonymous functions (or lambda terms)​

Referring to functions without giving them a name:

GHCi> (\ x -> x + 3) 4
7

GHCi> (\ list n -> take n (reverse list)) "hello" 3
"oll"

The \ is pronounced “lambda”.

Particularly useful as an argument to another function:

GHCi> map (\ x -> 3 * x + 1) [1,2,3]
[4,7,10]

Functions taking other functions as arguments are called higher-order functions.

We have learned about:

  • Expressions and values,
  • Functions and operators,
  • Numbers, characters, Booleans, lists (and strings),
  • Lambda terms.

In particular, try to get used to the function application syntax (using space) and the use of parentheses only for grouping.