Language Features Not Translatable into Lambda Calculus
That's not what "pure" means. Pure means, generally, that there is no mutable state, and no way to interact with the outside world (i.e. IO).

Haskell is not "directly translatable" (what translation between language features really means needs to be defined fist, but let's ignore that for now) into either the untyped or simply-typed lambda calculus. There are many features that prevent it from being so, but a good example is polymorphism: there is no polymorphic identity function in the simply-typed lambda calculus.

Instead, Haskell and SML are actually derived from the Hindley-Milner type system, which is an extension of the simply-typed lambda calculus with polymorphism and type-constructors (aka higher-kinded types). Hindley-Milner has the nice property that type inference is decidable without any type annotations.