For a (presumably) fully functional language, all we need is the following:

- Arrays
`Array#each`

- Function definitions and calls

We can define the two boolean constants `YES`

and `NO`

like this:

```
NO = []
YES = [ [] ]
```

An `IF`

function can be defined with `lambda`

, and uses `#each`

(which will only yield for `YES`

) to call the right branch:

```
IF =
lambda do |val, if_yes, if_no|
val.each { return if_yes.call }
if_no.call
end
```

With `IF`

, we can implement dyadic (“binary”) operations, such as `OR`

and `AND`

:

```
OR =
lambda do |val_a, val_b|
IF[val_a, -> { YES }, -> { IF[val_b, -> { YES }, -> { NO }] }]
end
```

This now works as expected:

```
p OR[YES, YES]
p OR[YES, NO]
p OR[NO, YES]
p OR[NO, NO]
```

We can define `ZERO`

and `ONE`

in the same way `NO`

and `YES`

:

```
ZERO = NO
ONE = YES
```

We can define successor function, which returns 1 more than the input:

```
SUCC =
lambda do |val|
[val]
end
```

We can define `TWO`

this way:

```
TWO = SUCC[ONE]
```

We can also define `PRED`

, which returns the previous integer in sequence:

```
PRED =
lambda do |val|
val.each { |e| return e }
return ZERO
end
```

With `PRED`

and `SUCC`

, we can define `SUM`

:

```
SUM =
lambda do |val_a, val_b|
IF[val_a, -> { SUM[PRED[val_a], SUCC[val_b]]}, -> { val_b }]
end
```

Now this works:

```
p SUM[ZERO, ZERO]
p SUM[ZERO, ONE]
p SUM[ZERO, TWO]
p SUM[TWO, TWO]
```

★★★