Introducing functions

In this page we will look at the basics of functions.

Examples

Here are some examples, in examples/functions.pf.

def

square(x) : x * x

parity(x) : 
    x % 2 == 0 : 
        "even"
    else : 
        "odd"

swap(x, y) : y, x

Basic syntax: defining functions

Functions are defined in the def section of the script.

The body of a Pipefish function consists of a single expression to be evaluated. There is no return keyword because all a Pipefish function ever does is return something, so it would be superfluous to have a keyword to point this out.

In Pipefish, a : precedes the body of a code block, which may be on the same line as the : so long as it fits into one line. Otherwise the block must also be marked off by indentation, as in the parity function.

Parentheses must be used to identify the parameters of a function.

Basic syntax: calling functions

Parentheses are not required when calling functions, except as necessary to establish precedence. Let's demonstrate in the REPL:

→ hub run "examples/functions.pf" as "Fun"                             
Starting script 'examples/functions.pf' as service 'Fun'.
Fun → parity 5
odd
Fun → swap "walrus", 42 
42, walrus  
Fun → square 3 + 4
49
Fun → (square 3) + 4 
13
Fun → square(3) + 4                                                    
13
Fun →

Conditionals

We have briefly met Pipefish's conditional operator, the example given being 2 + 2 == 3 : "Oops" ; 2 + 2 == 4 : "Whew!"; else : "Who even knows?", which evaluates to "Whew!".

In the body of a function, the ; can be replaced by a newline. So the following two functions are equivalent:

parity (x) : 
    x % 0 == 0 : 
        "even"
    else : 
        "odd"

parity (x) : 
    x % 0 == 0 : "even" ; else : "odd"

But the first is easier to read.

This becomes still more useful when we wish to nest conditionals, as demonstrated in examples/conditionals.pf:

classify (i int) :
    i > 0 :
        i < 10 : "small number"
        else : "big number"
    i == 0 :
        "zero"
    else :
        i > -10 : "small negative number"
        else : "big negative number"

Usually a conditional should have an else as the final condition, but there is one case in which it is useful to omit it, which we will discuss later.