Aha, thanks Csaba. So I’m not losing my marbles. The AST has a type signature of the “initial” but implements the “lowered”. So with -ddump-stg we can observe it: The version claimed in the type signature (returning a tuple): Foreign.it :: Foreign.C.Types.CDouble [GblId] = [] \u [] case ds_r1HA of { GHC.Types.D# ds2_s1HW [Occ=Once] -> case __pkg_ccall_GC main [ds2_s1HW GHC.Prim.realWorld#] of { (#,#) _ [Occ=Dead] ds4_s1I0 [Occ=Once] -> GHC.Types.D# [ds4_s1I0]; }; }; The final “lowered” version: Foreign.it :: Foreign.C.Types.CDouble [GblId] = [] \u [] case ds_r1HA of { GHC.Types.D# ds2_s1HW [Occ=Once] -> case __pkg_ccall_GC main [ds2_s1HW GHC.Prim.realWorld#] of { Unit# ds4_s1I0 [Occ=Once] -> GHC.Types.D# [ds4_s1I0]; }; }; Cheers! Chris