+++
title = "5c: Compiler Backend & Prettyprinter"
weight = 45
template = "doc.html"
+++

(To be fully documented)

## `++slog`

Deify printf

Prints stack trace `a` if the stack trace isn't null, then produces the other
input. `+slog` first takes `a`, producing a gate. The gate then takes some hoon.

#### Accepts

`a` is a `tang`, and is the sample of `+slog`.

`(slog a)` takes some `hoon`.

#### Produces

The result of the hoon given to `(slog a)`, with `a` printed to the terminal.

#### Source

```hoon
++  slog
  =|  pri=@
  |=  a=tang  ^+  same
  ?~(a same ~>(%slog.[pri i.a] $(a t.a)))
```

#### Examples

```
foo
> ((slog leaf+"foo" ~) (add 1 1))
2
```

#### Discussion

`slog` is intended to be put in the middle of a bunch of chained function
calls that string a piece of data through them, so that an error message will be
printed if there's one to print.

---

## `++mean`

Crash and printf

Ends the program and prints `a`, a tracing error message.

#### Accepts

`a` is a `tang`.

#### Produces

A crash, with `a` printed to the terminal.

#### Source

```hoon
++  mean
  |=  a=tang
  ^+  !!
  ?~  a  !!
  ~_(i.a $(a t.a))
```

#### Examples

```
> (mean leaf+"foo" ~)
foo
dojo: hoon expression failed
```

---