# OpaqueType::not-a-function


**Key:** `OpaqueType::not-a-function`

**Category:** arity · **Severity:** error

**Source:** [`src/diag/hints.nix:890`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L890)

the annotation on an opaque lambda must be a Pi type

## Example

`Hint::OpaqueType::not-a-function` means the `OpaqueType` position uses a non-function where the checker needs a Pi-typed term. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: an opaque lambda annotation must be a Pi type.
opaqueLam nat body

# Better: annotate it with a function type.
opaqueLam (pi nat (_: nat)) body

```


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