Hi there, in structural operational semantics, an evaluation context is often used to decompose an expression into a redex and its context. In a formal semantics on paper, an expression can just be "pattern matched" over the grammar of an evaluation context. If one wants to implement such a semantics in the form of an interpreter, I could not come up with a similarly nice solution. I have declared two separate data types (one for expressions, and one for evaluation contexts) and explicit functions to convert an expression into a (evaluation context, redex) pair. For example, I could have data Expr = Val Int | Plus Expr Expr and data Ctx = Hole | CPlusl Ctx Expr | CPlusr Int Expr Are there any tricks to mimick more closely what is going on in the formal semantics? Klaus