Surprises me that there's apparently so much Haskell code out there (in absolute terms), given its reputation as the archetypical "high barrier to entry" language.

Maybe the people who do get comfortable with it end up using it a _lot_.

Much of this is attributable to these rankings being counting stats rather than rate states, which makes age an asset. There's a surprising amount of Haskell in existence, but certainly if you're looking at rate at present Swift is growing far more quickly.