context

Source: /nix/store/mls72plk3raskl1r5afh3cl9ik3rn969-source/nix/context.nix

Module Description

Context manipulation utilities for attribute-based state

Combinators

has

Extracts a named attribute from a context.

A utility function for working with attrset-based contexts. In Nix, any attrset naturally “has” its attributes, so this is simply attribute access.

Type Signature

AttrSet -> String -> Value

Parameters

• ctx: An attrset context
• name: Name of the attribute to extract

Example

has { x = 42; y = 1; } "x"  # => 42

See Also

• put - Updates an attribute in context
• lift - Uses has/put implicitly for effect lifting

lift

Lifts an effect to work with larger contexts.

An effect requiring type A can run in any context that has an attribute containing A. This is the standard way to compose effects that work on different parts of a shared context.

Type Signature

String -> Fx<A, V> -> Fx<Ctx, V>

where Ctx is an attrset containing attribute name: A

Parameters

• name: The attribute name in the larger context
• e: Effect requiring the inner type

Example

runFx (
  provide { num = 10; other = "x"; } (
    lift "num"
      (pending (n: immediate n (n * 3)))
  )
)  # => 30

How It Works

Uses contraMap to:

1. Extract the named attribute for the inner effect
2. Update the named attribute with any state changes

See Also

• contraMap - Lower-level context transformation
• lens.zoomOut - Lens-based version

put

Updates a named attribute in a context.

Returns a new attrset with the specified attribute updated. If the attribute doesn’t exist, it is added.

Type Signature

AttrSet -> String -> Value -> AttrSet

Parameters

• ctx: The original context attrset
• name: Name of the attribute to update
• value: New value for the attribute

Example

put { x = 1; } "x" 42    # => { x = 42; }
put { x = 1; } "y" 2     # => { x = 1; y = 2; }

See Also

• has - Reads an attribute from context
• state.set - Sets the entire state, not just an attribute

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