Tyson Whitehead wrote:
and, in the -ddump-simpl we see the `go Left` branch returns a thunk on both the right and left sides that hold onto the evaluation of (x:xs,ys) as we would expect
``` Left x_aqy -> (GHC.Types.: @ a_a1q8 x_aqy (case ds1_d1rO of { (xs_aqz, ys_aqA) -> xs_aqz }), case ds1_d1rO of { (xs_aqz, ys_aqA) -> ys_aqA }); ```
These two expressions are selectors (accessing a field from a single constructor data type), which the compiler recoginizes (not sure where and how exactly) and implemented in terms of special stg_sel_<nnn> closures. At runtime, applying such a closure results in special THUNK_SELECTOR thunks. These thunks are evaluated by the garbage collector (see rts/sm/Scav.c). So this is a very special optimization in the garbage collector, not a flaw in your general mental model. Cheers, Bertram