# SigmaFst::refinement-failed


**Key:** `SigmaFst::refinement-failed`

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

**Source:** [`src/diag/hints.nix:916`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L916)

the first component violates the `fst` type's refinement predicate

## Example

`Hint::SigmaFst::refinement-failed` means the `SigmaFst` 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: fst has the right base type but fails its refinement.
pair (-1) proof

# Better: fst satisfies the refinement.
pair 1 proof

```


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