### ----------------------------------------------------
### -- WriteOnceHash
### -- Licenses: Artistic-2.0
### -- Authors: Elizabeth Mattijsen
### -- File: README.md
### ----------------------------------------------------
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NAME
====

WriteOnceHash - be a Hash in which each key can only be set once

SYNOPSIS
========

```raku
use WriteOnceHash;

# bind to predefined class
my %woh is WriteOnceHash;
%woh<a> = 5; # ok
%woh<a> = 1; # throws

CATCH {
    when X::Hash::WriteOnce {
        say "Sorry, already set {.key} before";
        .resume
    }
}

# mix in role on anything that does Associative
my %owoh{Any} does WriteOnce;
```

DESCRIPTION
===========

This module makes an `WriteOnceHash` class available that can be used instead of the normal `Hash`. The only difference with a normal `Hash` is, is that if an attempt is made to set the value of a key that **has already been set before**, an exception is thrown rather than just overwriting the key in the `Hash`.

Also binding to non-existing keys in the hash, and then assigning to the obtained container, will only work once. Iterating over values of the hash will only yield values, not containers (as a normal `Hash` would).

Also exports a `X::Hash::WriteOnce` error class that will be thrown if an attempt is made to set a key again.

The underlying `WriteOnce` role is also exported that can be used on objects that perform the `Associative` role that are not `Hash` (such as object hashes).

AUTHOR
======

Elizabeth Mattijsen <liz@raku.rocks>

Source can be located at: https://github.com/lizmat/WriteOnceHash . Comments and Pull Requests are welcome.

If you like this module, or what I’m doing more generally, committing to a [small sponsorship](https://github.com/sponsors/lizmat/) would mean a great deal to me!

COPYRIGHT AND LICENSE
=====================

Copyright 2018, 2020, 2021, 2024, 2025 Elizabeth Mattijsen

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.



