Core types (in particular CoreExpr) are not instances of Show. They are instances of Outputable, which allows them to be pretty printed. However, this pretty printing is good to view the structure of the expression that the CoreExpr represents, but doesn't show the structure of the CoreExpr itself.
Not sure how practical this would be, but to get an idea of the internal structure: if you can derive the SYB classes for the Core types (which would generally be useful if you do lots of traversals), you can easily define a generic show operation, which would give you a lot of control over what kind of output you'd like. There is an example here: http://hackage.haskell.org/trac/ghc/wiki/GhcApiAstTraversals look for the showData code - straighforward base plus lots of exceptions to avoid some ideosyncrasies of GHC's internal representations (in particular, don't try to traverse parts of the structure that haven't been filled yet) example output here http://hackage.haskell.org/packages/archive/ghc-syb/0.1.1/doc/html/GHC-SYB-U... hmm, I need to update that wiki page and the syb-utils package page - the code has moved from there, to the syb and ghc-syb packages, with proper maintainers. In particular, check the use of standalone deriving to get the SYB class instances for GHC API types in package ghc-syb: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ghc-syb Iirc, the syb instances for Core were not included, if that hasn't changed yet, it would be a useful addition (for an old first stab, see the link at the bottom of that wiki page, CoreSynTraverse). Claus