# SigmaSnd::refinement-failed


**Key:** `SigmaSnd::refinement-failed`

**Category:** refinement · **Severity:** error

**Source:** [`src/diag/hints.nix:918`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L918)

the second component violates the `snd` type's refinement predicate

## Example

`Hint::SigmaSnd::refinement-failed` means the `SigmaSnd` position contains a value that fails the refinement predicate. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: snd fails the refinement depending on fst.
pair 3 (-1)

# Better: snd satisfies the dependent refinement.
pair 3 4

```


---

[← All diagnostic hints](/nix-effects/diag-hints)