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We present the most precise measurement of the $z = 0$ HI mass function(HIMF) to date based on the final catalogue of the ALFALFA (Arecibo Legacy FastALFA) blind HI survey of the nearby Universe. The Schechter function fit has a`knee' mass $\log (M_{*}\,h^{2}_{70}/\mathrm{M_{\odot}}) = 9.94 \pm 0.01 \pm0.05$, a low-mass slope parameter $\alpha = -1.25 \pm 0.02 \pm 0.1$, and anormalisation $\phi_{*} = (4.5 \pm 0.2 \pm 0.8) \times 10^{-3} \;h^{3}_{70}\,\mathrm{Mpc^{-3}\,dex^{-1}}$, with both random and systematicuncertainties as quoted. Together these give an estimate of the HI content ofthe $z = 0$ Universe as $\Omega_{\mathrm{HI}} = (3.9 \pm 0.1 \pm 0.6) \times10^{-4} \, h^{-1}_{70}$ (corrected for HI self-absorption). Our analysis of theuncertainties indicates that the `knee' mass is a cosmologically fairmeasurement of the $z = 0$ value, with its largest uncertainty originating fromthe absolute flux calibration, but that the low-mass slope is onlyrepresentative of the local Universe. We also explore large scale trends in$\alpha$ and $M_{*}$ across the ALFALFA volume. Unlike with the 40 per centsample, there is now sufficient coverage in both of the survey fields to makean independent determination of the HIMF in each. We find a large discrepancyin the low-mass slope ($\Delta \alpha = 0.14 \pm 0.03$) between the tworegions, and argue that this is likely caused by the presence of a deep void inone field and the Virgo cluster in the other. Furthermore, we find that thevalue of the `knee' mass within the Local Volume appears to be suppressed by$0.18 \pm 0.04$ dex compared to the global ALFALFA value, which explains thelower value measured by the shallower HIPASS. We discuss possible explanationsand interpretations of these results and how they can be expanded on withfuture surveys.

The ALFALFA HI mass function: A dichotomy in the low-mass slope and a locally suppressed 'knee' mass