# SigmaFst::inhabitation-failed


**Key:** `SigmaFst::inhabitation-failed`

**Category:** inhabitation · **Severity:** error

**Source:** [`src/diag/hints.nix:912`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L912)

the first component does not inhabit the declared `fst` type

## Example

`Hint::SigmaFst::inhabitation-failed` means the `SigmaFst` position contains a value that does not inhabit the generated type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: the first component does not inhabit fst's type.
pair "not-a-nat" proof

# Better: fst inhabits its declared type.
pair (natLit 3) proof

```


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