Guarded Destructuring

In Goblin, you can destructure values into bindings while declaring:

• required fields (<req>)
• default values (:)
• fallback behavior (=>)

This is called Guarded Destructuring.

It ensures that:

• Missing values are always handled
• Required values are always enforced
• Defaults are always applied when provided
• Failures are always resolved explicitly

There are no unhandled states.

Basic Shape

a<req>, b:"x", c | expr => fallback

a<req>, b:"x", c | expr => fallback

What it does

1. Evaluates expr

2. Destructures positionally into bindings

3. Applies defaults:

   • Explicit defaults (b:"x")
   • Implicit fallback for unmarked bindings → ""

4. Checks required bindings (<req>)

5. If any required binding is nix:

   • The entire expression resolves to fallback

Example

filename<req>, size:"full", align:"center", caption | :split(body, "|") => "Invalid image syntax"

filename<req>, size:"full", align:"center", caption | :split(body, "|") => "Invalid image syntax"

Behavior

Equivalent Expansion

parts | :split(body, "|")

filename | parts[0]
size     | parts[1] ?? "full"
align    | parts[2] ?? "center"
caption  | parts[3] ?? ""

if filename.nix?
    return "Invalid image syntax"
xx

parts | :split(body, "|")

filename | parts[0]
size     | parts[1] ?? "full"
align    | parts[2] ?? "center"
caption  | parts[3] ?? ""

if filename.nix?
    return "Invalid image syntax"
xx

Binding Rules

Required (<req>)

filename<req>

filename<req>

• Must not be nix
• If nix → triggers fallback

Default (:)

size:"full"

size:"full"

• Used if value is nix
• Overrides implicit empty fallback

Unmarked Binding

caption

caption

If nix, becomes:

""

""

This is the implicit default.

Fallback (=>)

The => clause defines what happens when required bindings fail.

=> "<div>Bad input</div>"

=> "<div>Bad input</div>"

This can be:

• A value (used in output)
• An action (:say, logging, etc.)
• Any valid expression

Sheriff Rendering Example

filename<req>, size:"full", align:"center", caption | :split(body, "|") => """
<div class="sheriff-render-error">
  Image cannot be processed because of missing required fields
</div>
"""

filename<req>, size:"full", align:"center", caption | :split(body, "|") => """
<div class="sheriff-render-error">
  Image cannot be processed because of missing required fields
</div>
"""

If parsing fails, this HTML appears directly in the page.

Developer Feedback Example

x<req>, y<req> | coords => :say("You have to give all your numbers a value")

x<req>, y<req> | coords => :say("You have to give all your numbers a value")

• No crash
• No exception
• Controlled failure behavior

Multiple Required Fields

provider<req>, vid<req>, size:"full" | :split(body, "|") => "Invalid video"

provider<req>, vid<req>, size:"full" | :split(body, "|") => "Invalid video"

If any required binding is nix, fallback is used.

Provocation (Advanced)

Guarded destructuring ensures shape correctness.

If you need stricter guarantees (type, range, etc.), use provoke:

x<req>, y<req> | coords => "Bad input"

:provoke(x > 0)
:provoke(y > 0)

x<req>, y<req> | coords => "Bad input"

:provoke(x > 0)
:provoke(y > 0)

Destructuring handles presence.

provoke handles correctness.

Formatting Notes

Inline (preferred)

a<req>, b:"x", c | expr => fallback

a<req>, b:"x", c | expr => fallback

Tight form

a<req>,b:"x",c|expr=>fallback

a<req>,b:"x",c|expr=>fallback

Multiline fallback

a<req>, b | expr => """
Error: invalid input
"""

a<req>, b | expr => """
Error: invalid input
"""

How it differs

Why this exists

Goblin frequently works with structured text and dynamic output.

These require:

• strict shape enforcement
• safe defaults
• visible failures
• no silent breakage

Guarded Destructuring guarantees all of this in one expression.