Control flow

If

Usual if is a ifTrue: message for Boolean type, that takes codeblock as argument.
It will evaluate this block if it was send to true.
There is 4 messages with all possible combination.
The most common to use is ifTrue:ifFalse:

< 2 ifTrue: [1 echo]
> 2 ifFalse: [2 echo]
> 2 ifTrue: [3 echo] ifFalse: [4 echo]
< 2 ifFalse: [5 echo] ifTrue: [6 echo]

Try to say what numbers will be printed.

The last 2 messages have the signature:
Boolean ifTrue: [ -> T] ifFalse: [ -> T] -> T so they can be used as expressions
Which means that the last expression of each branch is returned as a result.
In the first example x will be equal 1:

x = true ifTrue: [1] ifFalse: [2]
y = 1 inc inc > 5 dec dec ifTrue: [1 + 5] ifFalse: [5 - 1]
// 3 > 3 
// y = 4

In this theoretical program we are getting content of the config file if it exists or create it if not.
Notice: codeblocks always return its last expression as value

Match

There was no way to do this in original Smalltalk, so they used to create a table with match-values to codeblocks

But since in niva I replaced inheritance with tagged unions I really need special syntax for matching.

| x // match against x
| 1 => x inc echo // if x == 1 then increment it and print
| 2 => x dec echo
|=> x echo // else\default branch

You can see that else branch, is just a usual branch where condition is missing
| 2 => x dec echo
| ??? => x echo
|=> x echo

x = "Alice"
// matching on x
| x
| "Bob" => "Hi Bob!"
| "Alice" => "Hi Alice!"
|=> "Hi guest"


// It can be used as expression as well
y = | x
    | "Bob" => "Hi Bob!"
    | "Alice" => "Hi Alice!"
    |=> "Hi guest"
y echo // "Hi Alice!" printed

Here example with unions, despite you will learn them a little bit later:

union Shape =
    | Rectangle width: Int height: Int
    | Circle    radius: Int
    
constructor Float pi = 3.14

// match on this(Shape)
Shape getArea -> Float = | this 
    | Rectangle => width * height |> toFloat
    | Circle => Float pi * radius * radius
    
// its exhaustive, so when u add new branch 
// all the matches will become errors until all cases processed

If syntax sugar

Since if is quite often thing, I created syntax sugar for ifTrue:
true => expr for true ifTrue: [expr]
and
true => expr |=> exprfor true ifTrue: [expr] ifTrue: [expr]

< 2 => "yay" echo
< 2 ifTrue: ["yay" echo]

// with else branch
> 2 => "yay" echo |=> "oh no" echo
> 2 ifTrue: ["yay" echo] ifFalse: ["oh no" echo]

You can find consistency between if and match.

| match againts => do conditiont => do

If abuse

There are no if ifelse else syntax chain, but you can do it like with ? : op in C

mark = "b"
score = mark == "a" => 10 |=>
    mark == "b" => 8 |=>
    mark == "c" => 6 |=>
    mark == "d" => 4
// score is 8 

I've never been in a situation where it would be necessary, prefer simple ifs and matches, this is for some very corner cases.

Last modified: 16 November 2024