# DPiFn::not-a-function


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

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

**Source:** [`src/diag/hints.nix:872`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L872)

the index selector `f` of `pi` must be a function `S -> I`

## Example

`Hint::DPiFn::not-a-function` means the `DPiFn` 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: the index selector is not a function.
descPi nat expectedIndex (_: descRet expectedIndex)

# Better: provide a selector from the domain into the index type.
descPi nat (x: expectedIndex) (_: descRet expectedIndex)

```


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