  The formyl radical HCO has been proposed as the basic precursor of many
complex organic molecules such as methanol (CH$_3$OH) or glycolaldehyde
(CH$_2$OHCHO). Using ALMA, we have mapped, for the first time at high angular
resolution ($\sim$1$^{\prime\prime}$, $\sim$140 au), HCO towards the Solar-type
protostellar binary IRAS 16293$-$2422, where numerous complex organic molecules
have been previously detected. We also detected several lines of the chemically
related species H$_2$CO, CH$_3$OH and CH$_2$OHCHO. The observations revealed
compact HCO emission arising from the two protostars. The line profiles also
show redshifted absorption produced by foreground material of the circumbinary
envelope that is infalling towards the protostars. Additionally, IRAM 30m
single-dish data revealed a more extended HCO component arising from the common
circumbinary envelope. The comparison between the observed molecular abundances
and our chemical model suggests that whereas the extended HCO from the envelope
can be formed via gas-phase reactions during the cold collapse of the natal
core, the HCO in the hot corinos surrounding the protostars is predominantly
formed by the hydrogenation of CO on the surface of dust grains and subsequent
thermal desorption during the protostellar phase. The derived abundance of HCO
in the dust grains is high enough to produce efficiently more complex species
such as H$_2$CO, CH$_3$OH, and CH$_2$OHCHO by surface chemistry. We found that
the main formation route of CH$_2$OHCHO is the reaction between HCO and
CH$_2$OH.

Astrophysics of Galaxies