Hello: CUE:

name: inverse
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<p>Hello: "CUE" </p>

## anarcher@gmail.com
## 2020-09-01

---
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![logo](./cue01.png)

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# CUE

- Unifies data and schema

- Command-line tool

- Logic engine

- Rich Go APIs and integration

- https://cuelang.org

---

# Values in CUE

- CUE is a superset of JSON.
- https://cuelang.org/docs/tutorials/tour/intro/json/

```cue
seoul: {
    name: "Seoul"
    pop:  9.7M
    capital: true
}
```

```json
"seoul" : {
    "name" : "Seoul"
    "pop" : 9700000
    "capital": true
}
```

---

# Types are values

- https://cuelang.org/docs/tutorials/tour/intro/cue/

```cue
// cue
municipality: {
    name: string
    pop: int
    capital: bool
}
```

```go
// go
type municipality struct {
    Name string
    Pop int
    Capital bool
}
```

---

# Types & Constraints
- https://cuelang.org/docs/tutorials/tour/intro/constraints/

```cue
largeCapital: {
  name:    string
  pop:     >5M
  capital: true
}
```

```cue
schema: {
    name:  string
    age:   int
    human: true // always true
}

viola: schema
viola: {
    name: "Viola"
    age:  38
}
```

---

# Definitions

- https://cuelang.org/docs/tutorials/tour/intro/schema/
- starts with a `#` or `_#`
- use for validation, not output

```cue
#Conn: {
    address:  string
    port:     int
    protocol: string
    // ...    // uncomment this to allow any field
}

lossy: #Conn & {
    address:  "1.2.3.4"
    port:     8888
    protocol: "udp"
    // foo: 2 // uncomment this to get an error
}
```

---

# Modules, Packages, and Instances
- https://cuelang.org/docs/concepts/packages/

```sh
cue.mod
|-- module.cue  // The module file 
|-- pkg         // copies of external packages
|-- gen         // files generated from external sources
|-- usr         // user-defined constraints
```

```cue
root                    // must contain:
|-- cue.mod
|   |-- module.cue      // module: "example.com"
|-- schemas
|   |-- trains
|   |   |-- track.cue   // package track
...
|-- data.cue  
    // import "example.com/schemas/trains:track"
    <module identifier>/<relative position of package within module>:<package name>

```

---
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# Modules, Packages, and Instances
- https://cuelang.org/docs/concepts/packages/

Within a module, all .cue files with the same package name are part of the same package.

Instance of a package:
- only the files belonging to that package in that directory and its ancestor directories 
    
Example:
 - module root: `schema`
 - medial directories: `policy`
 - leaf directories: `data`

---
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# Default value
- https://cuelang.org/docs/tutorials/tour/types/defaults/

```cue
// defaults.cue
// any positive number, 1 is the default
replicas: uint | *1

// the default value is ambiguous
protocol: *"tcp" | "udp"
protocol: *"udp" | "tcp"
```

```cue
$ cue eval defaults.cue
replicas: 1
protocol: "tcp" | "udp" | *_|_
```

---
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```cue
// Conditional Fields
price: number
// Require a justification if price is too high
if price > 100 {
    justification: string
}
price: 200

// List Comprehensions
[ for x in #items if x rem 2 == 0 { x*x } ]
#items: [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

// Field Comprehensions
import "strings"
#a: [ "Barcelona", "Shanghai", "Munich" ]
for k, v in #a {
    "\( strings.ToLower(v) )": {
        pos:     k + 1
        name:    v
        nameLen: len(v)
    }
}

```

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# Builtin packages

- CUE has a collection of builtin packages that are compiled into the `cue` binary.
- https://pkg.go.dev/cuelang.org/go/pkg?tab=doc
- `tool/exec.Run`: https://pkg.go.dev/cuelang.org/go@v0.2.2/pkg/tool/exec?tab=doc
- Not yet: user defined native packages...

# `cue` commands
https://github.com/cuelang/cue/blob/master/doc/cmd/cue.md

`cmd` executes user-defined named commands for each of the listed instances.
- Commands are defined in cue files ending with `_tool.cue`
- Command definition: https://github.com/cuelang/cue/blob/master/doc/cmd/cue.md#command-definition

---
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# `cue` commands

<pre><code class="remark-code" style="font-size: 80%;">// hello.cue
package foo

city: "Seoul"
who: *"World" | string @tag(who)
</code></pre>

<pre><code class="remark-code" style="font-size: 80%;">// hello_tool.cue
package foo
import "tool/exec"

command: hello: {
    print: exec.Run & {
	    cmd: "echo Hello \(who)! Welcome to \(city)."
    }
}
</code></pre>

<pre><code class="remark-code" style="font-size: 80%;">// hello_tool.cue
$ cue --inject who=Jan hello
Hello Jan! Welcome to Seoul.
$ cue hello
Hello World! Welcome to Seoul.
</code></pre>

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.interstitial[![](/show-of-hands.png)]

---
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Thank you