I'd probably prefer using a fold over explicit recursion. This gets easier if
you split your code into two steps - one that does just one level of lookup,
and one that does nested lookups. Additionally, I'd prefer list indexes rather
than pair lookup functions:
(define (data/ref s key)
(cond
[(hash? s) (hash-ref s key)]
[(list? s) (list-ref s key)]
[else fail-somehow ...]))
(define (data/fetch s keys)
(define (fetch-once key s)
(data/ref s key))
(foldl fetch-once s keys))
> (data/ref (hash 'a 1 'b 2) 'a)
1
> (data/ref '(a b c) 2)
'c
> (data/fetch (hash 'a '(foo bar baz) 'b 0) '(a 2))
'baz
By splitting the recursion (define (data/ref s key)
(cond
[(hash? s) (hash-ref s key)]
[(list? s) (list-ref s key)]
[else fail-somehow ...]))
(define (data/fetch s keys)
(define (fetch-once key s)
(data/ref s key))
(foldl fetch-once s keys))
> (data/ref (hash 'a 1 'b 2) 'a)
1
> (data/ref '(a b c) 2)
'c
> (data/fetch (hash 'a '(foo bar baz) 'b 0) '(a 2))
'baz
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