# DArgBody::not-a-desc


**Key:** `DArgBody::not-a-desc`

**Category:** description · **Severity:** error

**Source:** [`src/diag/hints.nix:846`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L846)

the body of `arg` must produce a description (Desc I), not an ordinary value. Build one with `descRet`, `descArg`, `descRec`, `descPi`, or `descPlus`.

## Example

`Hint::DArgBody::not-a-desc` means the `DArgBody` position uses an ordinary term where the checker needs a Desc. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: the body returns an ordinary term.
descArg nat (_: natLit 0)

# Better: the body returns a description.
descArg nat (_: descRet tt)

```


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