(compile nil `(lambda () (declare (ftype (function (t) fixnum) x)))) => The function COMPILER::TYPE-FILTER is undefined. which stems from http://common-lisp.net/gitweb?p=projects/mkcl/mkcl.git;a=blob;f=src/cmp/cmpenv.lsp;h=11c437566969e85449a57bf95ac5e1841e518535;hb=HEAD#l94 missing the package prefix. -- With best regards, Stas.