# AppHead::not-a-function


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

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

**Source:** [`src/diag/hints.nix:882`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L882)

the head of an application must have a function type (Pi)

## Example

`Hint::AppHead::not-a-function` means the `AppHead` 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 application head is not a function.
app (natLit 0) (natLit 1)

# Better: apply a lambda or another Pi-typed term.
app (lam "x" nat (x: x)) (natLit 1)

```


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