provide.nix

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

Module Description

Context provision and manipulation

Combinators

func

Creates an effect from a function on state.

This is a convenience combinator that reads the state and applies a pure function to produce the value. Equivalent to map f state.get.

Type Signature

(S -> V) -> Fx<S, V>

Parameters

• f: A pure function from state to value

Example

provide 10 (func (n: n * 2))  # => 20
provide { x = 5; } (func (ctx: ctx.x + 1))  # => 6

See Also

• state.get - Just reads state without transformation
• map - Transforms value of an existing effect

provide

Satisfies a context requirement, eliminating it.

This is the primary way to “run” effects - by providing the context they need. The provided value becomes the context for the inner effect, and the resulting effect has no requirements for that context.

Type Signature

S -> Fx<S, V> -> Fx<T, V> (for any T)

Parameters

• s: The context value to provide
• e: Effect that requires context S

Example

# Effect needing a number
double = pending (n: immediate n (n * 2))

# Provide the number to get a runnable effect
runFx (provide 21 double)  # => 42

# Provide a record context
runFx (provide { x = 5; y = 3; }
  (pending (ctx: immediate ctx (ctx.x + ctx.y))))  # => 8

See Also

• provideLeft - Provides left side of paired context
• providePart - Provides part using custom getter/setter

provideLeft

Provides the left component of a paired context requirement.

When an effect requires {fst: A, snd: B}, this provides the A part, leaving an effect that only requires B.

Type Signature

A -> Fx<{fst: A, snd: B}, V> -> Fx<B, V>

Parameters

• a: Value for the left/first context component
• e: Effect requiring paired context

Example

provide 20 (
  provideLeft 10
    (pending (ctx: immediate ctx (ctx.fst + ctx.snd)))
)  # => 30

See Also

• provide - Provides entire context
• andNil - Extends context given unit on right

providePart

Provides part of a context using custom getter/setter.

A generalized version of provide that allows partial satisfaction of context requirements using custom lensing functions.

Type Signature

A -> (A -> B -> S) -> (B -> S -> B) -> Fx<S, V> -> Fx<B, V>

Parameters

• a: Value to inject into context
• cmap: Combines A and remaining context B to form full context S
• fmap: Extracts remaining context from result state
• e: Effect requiring full context S

Example

provide 5 (
  providePart 10
    (a: b: { x = a; y = b; })
    (_b: ctx: ctx.y)
    (pending (ctx: immediate ctx (ctx.x + ctx.y)))
)  # => 15

See Also

• provide - Simpler full context provision
• provideLeft - Standard paired context provision

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