result

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

Module Description

Simple Result-based error handling built on condition system.

Provides familiar throw/catch semantics as a convenience layer over the more powerful condition system. Unlike conditions, Results always unwind the stack on error.

Comparison

Example

result = catch' (
  divide = x: y:
    if y == 0
    then throw' "DivideByZero" { dividend = x; }
    else pure (x / y);
  
  divide 10 2
);
# => Ok 5

result2 = catch' (divide 10 0);
# => Err { type = "DivideByZero"; dividend = 10; }

When to Use

• Use Result for simple error propagation
• Use Conditions when you need resumption or multiple handlers

Combinators

bindResult

Chain Result-producing operations.

If the Result is successful, applies the function to the value. Otherwise passes the error through.

Type Signature

(A -> Result<B>) -> Result<A> -> Result<B>

Parameters

• f: Function returning a Result
• result: Input Result

Example

validateAge = age:
  if age < 0 then { success = false; error = "negative"; }
  else { success = true; value = age; };

bindResult validateAge { success = true; value = 25; }
# => { success = true; value = 25; }

catch'

Catch errors thrown with throw’.

Wraps an effect to catch any thrown errors and return them as Result values: { success = true; value = v; } for success or { success = false; error = e; } for errors.

Type Signature

Fx<S, V> -> Fx<S, Result<V>>

Parameters

• effect: Effect that may throw errors

Returns

Result with either:

• { success = true; value = V; }
• { success = false; error = ErrorDetails; }

Example

result = catch' (
  bind (pure 10) (x:
    if x > 5
    then throw' "TooBig" { value = x; }
    else pure x
  )
);
# => { success = false; error = { type = "TooBig"; value = 10; }; }

See Also

• throw' - Throw an error
• try - Catch with default value

mapResult

Transform the value inside a successful Result.

Applies a function to the value if the Result is successful, otherwise passes the error through unchanged.

Type Signature

(A -> B) -> Result<A> -> Result<B>

Parameters

• f: Function to apply to successful value
• result: Result to transform

Example

mapResult (x: x * 2) { success = true; value = 21; }
# => { success = true; value = 42; }

mapResult (x: x * 2) { success = false; error = {...}; }
# => { success = false; error = {...}; }

throw'

Throw an error that unwinds the stack.

Signals an error condition that cannot be resumed. The error will propagate up to the nearest catch’ handler.

Type Signature

String -> Attrs -> Fx<S, never>

Parameters

• errorType: Error type identifier
• details: Additional error information

Example

validateAge = age:
  if age < 0
  then throw' "InvalidAge" { age = age; }
  else pure age;

See Also

• catch' - Handle thrown errors
• try - Catch with default value
• error - Non-resumable condition system error

try

Try an effect, returning a default value on error.

Convenience wrapper around catch’ that extracts the value or returns the default on error.

Type Signature

V -> Fx<S, V> -> Fx<S, V>

Parameters

• default: Value to return if effect throws
• effect: Effect to try

Example

result = try 0 (
  if somethingWrong
  then throw' "Failed" {}
  else pure 42
);
# => 42 on success, 0 on error

See Also

• catch' - Catch and inspect error
• throw' - Throw an error

Generated from /nix/store/mls72plk3raskl1r5afh3cl9ik3rn969-source/nix/result.nix