# AppArg::type-mismatch


**Key:** `AppArg::type-mismatch`

**Category:** type-mismatch · **Severity:** error

**Source:** [`src/diag/hints.nix:884`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L884)

the argument does not match the function's domain

## Example

`Hint::AppArg::type-mismatch` means the `AppArg` position supplies a term whose type does not match the expected type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: the argument does not match the function domain.
app (lam "x" nat (x: x)) true_

# Better: pass an argument from the domain.
app (lam "x" nat (x: x)) (natLit 1)

```


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