> For the complete documentation index, see [llms.txt](https://fluence.gitbook.io/aqua-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://fluence.gitbook.io/aqua-book/language/expressions/overrideable-constants.md).

# Overrideable constants

Static configuration pieces that affect compilation

### `const`

Constant definition.

Constants can be used all across functions, exported and imported. If a constant is defined using `?=` , it can be overridden by value via compiler flags or imported values.

```haskell
-- This can be overridden with -const "TARGET_PEER_ID = \"other peer id\""
const TARGET_PEER_ID ?= "this is a target peer id"

-- This constant cannot be overridden
const SERVICE_ID = "service id"
```

You can assign only literals to constants. Constant type is the same as literal type. You can override only with a subtype of that literal type.