# ::unhandled-lam


**Key:** `::unhandled-lam`

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

**Source:** [`src/diag/hints.nix:939`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L939)

this lambda has no inference rule — its domain and codomain types are not determined by its syntax. Annotate with `ann (lam …) (pi A B)` to drive checking, or place it where the surrounding rule already supplies an expected Pi type.

## Example

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

```nix
# Bad: a lambda reaches inference without an expected Pi type.
infer emptyCtx (lam "x" nat (x: x))

# Better: annotate it so checking mode knows the Pi type.
infer emptyCtx (ann (lam "x" nat (x: x)) (pi nat (_: nat)))

```


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