# 2a: Unit Logic
## `+biff`
A [`+unit`](https://docs.urbit.org/hoon/1c#unit) as argument.
Applies a `$gate` `.b` that produces a [`+unit`](https://docs.urbit.org/hoon/1c#unit) to the unwrapped value of [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.a` (`.u.a`). If `.a` is empty, `~` is produced.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a `$gate` that accepts a `$noun` and produces a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Source
```hoon
++ biff
|* [a=(unit) b=$-(* (unit))]
?~ a ~
(b u.a)
```
#### Examples
```
> (biff (some 5) |=(a=@ (some (add a 2))))
[~ u=7]
```
```
> (biff ~ |=(a=@ (some (add a 2))))
~
```
***
## `+bind`
Non-unit `$gate` to [`+unit`](https://docs.urbit.org/hoon/1c#unit), producing [`+unit`](https://docs.urbit.org/hoon/1c#unit).
Applies a `$gate` `.b` to the value (`.u.a`) of a [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.a`, producing a [`+unit`](https://docs.urbit.org/hoon/1c#unit). Used when you want a `$gate` that does not accept or produce a [`+unit`](https://docs.urbit.org/hoon/1c#unit) to both accept and produce a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a gate.
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Source
```hoon
++ bind
|* [a=(unit) b=gate]
?~ a ~
[~ u=(b u.a)]
```
#### Examples
```
> (bind ((unit @) [~ 97]) ,@t)
[~ u='a']
```
```
> =a |=(a=@ (add a 1))
> (bind ((unit @) [~ 2]) a)
[~ u=3]
```
***
## `+bond`
Replace null.
Replaces an empty [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.b` with the product of a called `+trap` `.a`. If the [`+unit`](https://docs.urbit.org/hoon/1c#unit) is not empty, then the original [`+unit`](https://docs.urbit.org/hoon/1c#unit) is produced.
#### Accepts
`.a` is a `+trap`.
`.b` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
Either the product of `.a` or the value inside of [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.b`.
#### Source
```hoon
++ bond
|* a=(trap)
|* b=(unit)
?~ b $:a
u.b
```
#### Examples
```
> (bex 10)
1.024
> ((bond |.((bex 10))) ~)
1.024
```
```
> ((bond |.((bex 10))) (slaw %ud '123'))
123
```
***
## `+both`
Group [`+unit`](https://docs.urbit.org/hoon/1c#unit) values into pair.
Produces `~` if either `.a` or `.b` are empty. Otherwise, produces a [`+unit`](https://docs.urbit.org/hoon/1c#unit) whose value is a cell of the values of two input [`+unit`](https://docs.urbit.org/hoon/1c#unit)s `.a` and `.b`.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit) of the two initial values.
#### Source
```hoon
++ both
|* [a=(unit) b=(unit)]
?~ a ~
?~ b ~
[~ u=[u.a u.b]]
```
#### Examples
```
> (both (some 1) (some %b))
[~ u=[1 %b]]
```
```
> (both ~ (some %b))
~
```
***
## `+clap`
Combine two [`+unit`s](https://docs.urbit.org/hoon/1c#unit) with a gate.
Applies a binary `$gate` `.c` (which does not usually accept or produce a [`+unit`](https://docs.urbit.org/hoon/1c#unit)) to the values of two [`+unit`s](https://docs.urbit.org/hoon/1c#unit), `.a` and `.b`, producing a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
If `.a` is null, produces `.b`. If `.b` is null, produces `.a`. Otherwise, produces the product of `.c` in a `+unit`.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.c` is a `$gate` that performs a binary operation.
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Source
```hoon
++ clap
|* [a=(unit) b=(unit) c=_=>(~ |=(^ +<-))]
?~ a b
?~ b a
[~ u=(c u.a u.b)]
```
#### Examples
```
> (clap ~ `'b' |=([a=@tD b=@tD] `tape`[a b ~]))
[~ u='b']
```
```
> (clap `'a' ~ |=([a=@tD b=@tD] `tape`[a b ~]))
[~ u='a']
```
```
> (clap `'a' `'b' |=([a=@tD b=@tD] `tape`[a b ~]))
[~ u="ab"]
```
***
## `+clef`
Compose two [`+unit`s](https://docs.urbit.org/hoon/1c#unit) with a gate.
Applies a binary `$gate` `.c` (which does not usually accept [`+unit`](https://docs.urbit.org/hoon/1c#unit)s) to the values of two [`+unit`](https://docs.urbit.org/hoon/1c#unit)s, `.a` and `.b`, producing a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
If `.a` or `.b` are null, produces null. Otherwise, produces the produce of `.c`.
#### Accepts
`.a` is a `+unit`.
`.b` is a `+unit`.
`.c` is a `$gate` that takes a `+cell` and produces a `+unit`.
#### Produces
A `+unit`.
#### Source
```hoon
++ clef
|* [a=(unit) b=(unit) c=_=>(~ |=(^ `+<-))]
?~ a ~
?~ b ~
(c u.a u.b)
```
#### Examples
```
> (clef ~ `'b' |=([a=@tD b=@tD] `(unit tape)`[~ [a b ~]]))
~
```
```
> (clef `'a' ~ |=([a=@tD b=@tD] `(unit tape)`[~ [a b ~]]))
~
```
```
> (clef `'a' `'b' |=([a=@tD b=@tD] `(unit tape)`[~ [a b ~]]))
[~ "ab"]
```
***
## `+drop`
Convert a [`+unit`](https://docs.urbit.org/hoon/1c#unit) to a list.
Makes a [`+list`](https://docs.urbit.org/hoon/1c#list) of the unwrapped value (`.u.a`) of a [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.a`.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
A list.
#### Source
```hoon
++ drop
|* a=(unit)
?~ a ~
[i=u.a t=~]
```
#### Examples
```
> =a ((unit @) [~ 97])
> (drop a)
[i=97 t=~]
```
```
> =a ((unit @) [~])
> (drop a)
~
```
***
## `+fall`
Give [`+unit`](https://docs.urbit.org/hoon/1c#unit) a default value.
Produces a default value `.b` for a [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.a` in cases where `.a` is null.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a `$noun` that's used as the default value.
#### Produces
Either a `$noun` `.b` or the unwrapped value of [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.a`.
#### Source
```hoon
++ fall
|* [a=(unit) b=*]
?~(a b u.a)
```
#### Examples
```
> (fall ~ 'a')
'a'
> (fall [~ u=0] 'a')
0
```
***
## `+flit`
Make filter.
Accepts `$gate` `.a` which produces a `$flag`. Applies `.a` to `.b`, producing `.b` wrapped in a `+unit` if true, otherwise produces null.
`+flit` is a wet `$gate` that takes `.a` and produces a new wet `$gate` that takes `.b`.
#### Accepts
`.a` is a `$gate` which produces a `$flag`, and is the sample of the outer wet gate.
`.b` is any `$noun`, and is the sample of the inner wet gate.
#### Produces
A `(unit [type])`, where `[type]` is the type of `.b`.
#### Source
```hoon
++ flit
|* a=$-(* ?)
|* b=*
?.((a b) ~ [~ u=b])
```
#### Examples
```
> =f (flit |=(=@ud ?:(=(ud 1) %.y %.n)))
> (f 1)
[~ 1]
> (f 2)
~
```
***
## `+hunt`
First of units.
Apply binary `$gate` `.ord`, which produces a `$flag`, to units `.a` and `.b`. Produce `.a` if true and `.b` if false. If `.a` is null, produce `.b`. If `.b` is null, produce `.a`.
This allows selecting between two units by some rule.
#### Accepts
`.ord` is a `$-(^ ?)` - a binary `$gate` that produces a `$flag`.
`.a` is a `+unit`.
`.b` is a `+unit`.
#### Produces
A `+unit`.
#### Source
```hoon
++ hunt
|* [ord=$-(^ ?) a=(unit) b=(unit)]
^- %- unit
$? _?>(?=(^ a) u.a)
_?>(?=(^ b) u.b)
==
?~ a b
?~ b a
?:((ord u.a u.b) a b)
```
#### Examples
```
> (hunt gte ~ `20)
[~ 20]
```
```
> (hunt gte `10 ~)
[~ 10]
```
```
> (hunt gte `10 `20)
[~ 20]
```
***
## `+lift`
Curried bind.
Accepts `$gate` `.a` and produces a `$gate` that accepts [`+unit`](https://docs.urbit.org/hoon/1c#unit) `.b` to which it applies `.a`. Used when you want a `$gate` that does not accept or produce a [`+unit`](https://docs.urbit.org/hoon/1c#unit) to both accept and produce a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Accepts
`.a` is a `$mold`.
`.b` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Source
```hoon
++ lift
|* a=mold
|* b=(unit)
(bind b a)
```
#### Examples
```
> ((lift dec) `(unit @)`~)
~
```
```
> ((lift dec) `(unit @)`[~ 20])
[~ 19]
```
***
## `+mate`
Choose.
Accepts two [`+unit`s](https://docs.urbit.org/hoon/1c#unit) `.a` and `.b` whose values are expected to be equivalent. If either is empty, then the value of the other is produced. If neither are empty, it asserts that both values are the same and produces that value. If the assertion fails, `+mate` crashes with `'mate'` in the stack trace.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
`.b` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit) or crash.
#### Source
```hoon
++ mate
|* [a=(unit) b=(unit)]
?~ b a
?~ a b
?.(=(u.a u.b) ~>(%mean.'mate' !!) a)
```
#### Examples
```
> =a ((unit @) [~ 97])
> =b ((unit @) [~ 97])
> (mate a b)
[~ 97]
```
```
> =a ((unit @) [~ 97])
> =b ((unit @) [~])
> (mate a b)
[~ 97]
```
```
> =a ((unit @) [~ 97])
> =b ((unit @) [~ 98])
> (mate a b)
! 'mate'
! exit
```
***
## `+need`
Unwrap [`+unit`](https://docs.urbit.org/hoon/1c#unit).
Retrieve the value from a [`+unit`](https://docs.urbit.org/hoon/1c#unit) and crash if the [`+unit`](https://docs.urbit.org/hoon/1c#unit) is null.
#### Accepts
`.a` is a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Produces
Either the unwrapped value of `.a` (`.u.a`), or crash.
#### Source
```hoon
++ need
~/ %need
|* a=(unit)
?~ a ~>(%mean.'need' !!)
u.a
```
#### Examples
```
> =a ((unit [@t @t]) [~ ['a' 'b']])
> (need a)
['a' 'b']
```
```
> =a ((unit @ud) [~ 17])
> (need a)
17
```
```
> =a ((unit @) [~])
> (need a)
! exit
```
***
## `+some`
Wrap value in a [`+unit`](https://docs.urbit.org/hoon/1c#unit).
Takes any `$noun` `.a` and produces a [`+unit`](https://docs.urbit.org/hoon/1c#unit) with the value set to `.a`.
#### Accepts
`.a` is a `$noun`.
#### Produces
A [`+unit`](https://docs.urbit.org/hoon/1c#unit).
#### Source
```hoon
++ some
|* a=*
[~ u=a]
```
#### Examples
```
> (some ['a' 'b'])
[~ u=['a' 'b']]
```
```
> (some &)
[~ u=%.y]
```
***