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### Nickel configuration with SemanticVersion contract

Source: https://nickel-lang.org

Interactive Nickel configuration example with an intentional error in the version field. The SemanticVersion contract uses a regex pattern and std.contract.blame_with_message to report a validation failure. Run it in the REPL to see the error message.

```nickel
let conf = {
name = "NiCl",
version = "0.0.1$",
description = "My cool app!"
} in
let SemanticVersion = fun label value =>
let pattern = "^\\d{1,2}\\.\\d{1,2}(\\.\\d{1,2})?$"
in
if std.string.is_match pattern value then
value
else
let msg = "invalid version number" in
std.contract.blame_with_message msg label
in
let AppSchema = {
name | String,
version | SemanticVersion,
description | String,
} in
conf | AppSchema
```

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