[Haskell-cafe] Why doesn't this consume all the computer's memory?

[Bertram Felgenhauer]

Bertram Felgenhauer via Haskell-Cafe wrote:
> 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).

Or rather  rts/sm/Evac.c, which has functions unchain_thunk_selectors()
and eval_thunk_selector() that do the actual work.

Cheers,

Bertram