# PiDom::not-a-type


**Key:** `PiDom::not-a-type`

**Category:** sort · **Severity:** error

**Source:** [`src/diag/hints.nix:858`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L858)

the domain of Π must be a type (live in some U(k)), not a term or value. Supply a type expression like `nat`, `bool`, `u 0`, or a user-defined datatype.

## Example

`Hint::PiDom::not-a-type` means the `PiDom` position uses a term or value where the checker needs a type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: a Pi domain must be a type, not a term.
pi (natLit 0) (_: nat)

# Better: put a type in the domain.
pi nat (_: nat)

```


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