Conal Elliott
I share your perspective, Edsko. If foo and (Let foo id) are indistinguishable to clients of your module and are equal with respect to your intended semantics of Exp, then I'd say at least this one monad law holds. - Conal
I am at least sympathetic to this perspective, but the Expr constructors are not as polymorphic as the monad operations: if in do a <- foo return a foo has type "ExprM String" (perhaps foo is equal to "return []"), then we want to generate the DSL expression "Let [] id", but "[]" is not of type "Expr". Because whenever foo's type is not "ExprM Expr" the above code using do notation must be exactly equal to foo, by parametricity even when foo's type is "ExprM Expr" we cannot generate Let. -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig The pomotarians have nothing to lose but their value chains. Call the Thermodynamics Police!