but only if any deviation from purity requires special syntax at the call site.
Haskell makes it easy to write data and function types that enforce purity at the call site, throwing a type error if you make a mistake. Not just purity though, you can restrict these types to any arbitrary set of methods of your choosing. This lets you do things like parsing a blob of JSON and having all functions that depend on the result be guaranteed not to have to deal with a failed parse or otherwise invalid data. The fact that the data is good has been encoded in the types, preventing you from passing bad data by throwing a type error.
Haskell makes it easy to write data and function types that enforce purity at the call site, throwing a type error if you make a mistake. Not just purity though, you can restrict these types to any arbitrary set of methods of your choosing. This lets you do things like parsing a blob of JSON and having all functions that depend on the result be guaranteed not to have to deal with a failed parse or otherwise invalid data. The fact that the data is good has been encoded in the types, preventing you from passing bad data by throwing a type error.