My favorite solution to the expression problem is the tagless-final style. It can be implemented in both ML-likes and Haskell-likes leveraging either expressive module systems or type class systems to combine the benefits of both shallow and deep embedding of DSL's. It requires one to bend their mind to really understand but once it clicks it is extremely powerful. Perhaps that "click" is the mind snapping once sufficiently bent.

http://okmij.org/ftp/tagless-final/

"The so-called ``tagless-final'' style is a method of embedding domain-specific languages (DSLs) in a typed functional host language such as Haskell, OCaml, Scala or Coq. It is an alternative to the more familiar embedding as a (generalized) algebraic data type. It is centered around interpreters: Evaluator, compiler, partial evaluator, pretty-printer, multi-pass optimizer are all interpreters of DSL expressions. Doing a tagless-final embedding is literally writing a denotational semantics for the DSL -- in a host programming language rather than on paper."