# ::unhandled-pair


**Key:** `::unhandled-pair`

**Category:** shape · **Severity:** error

**Source:** [`src/diag/hints.nix:941`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L941)

this pair has no inference rule — its component types are not determined by its syntax. Annotate with `ann (pair …) (sigma A B)` to drive checking, or place it where the surrounding rule already supplies an expected Sigma type.

## Example

`Hint::unhandled-pair` means a pair is in inference mode without an expected Sigma type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: a pair reaches inference without an expected Sigma type.
infer emptyCtx (pair (natLit 1) true_)

# Better: annotate it with the Sigma type it should inhabit.
infer emptyCtx (ann (pair (natLit 1) true_) (sigma "x" nat (_: bool)))

```


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