# MuDesc::not-a-desc


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

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

**Source:** [`src/diag/hints.nix:864`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L864)

the description argument of μ must be a Desc I term, not an ordinary value. Construct it with `descRet`, `descArg`, `descRec`, `descPi`, or `descPlus`.

## Example

`Hint::MuDesc::not-a-desc` means the `MuDesc` 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: mu expects a Desc, not a term.
mu nat (natLit 0)

# Better: construct the Desc explicitly.
mu nat (descRet zero)

```


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