I'm trying to write a parser for a CSV file with optional columns.
Simplified version: There are 2 mandatory columns, after which there can
be 0+ 4-column groups describing a person. Each group has the same column
headers.
; legal column arrangements:
RequiredA RequiredB
RequiredA RequiredB Name Age First Last
RequiredA RequiredB Name Age First Last Name Age First Last
; illegal: if an optional group is present, it must have all 4 columns
RequiredA RequiredB Name Age First Last Name
I thought I could do this straightforwardly with `match`, but I'm wrong.
Can someone point me to the way to write such a match clause?
Various failed attempts:
(list reqA reqB (opt1 opt2 opt3 opt4) ...) ; syntax error. matching
clauses do not do grouping like this
(list reqA reqB (list opt1 opt2 opt3 opt4) ...) ; didn't expect this to
work since it would specify an embedded list. I was right.
This one surprised me:
(match row
[(list required1 required2 (and opt1 opt2 opt3 opt4) ...)
(list opt1 opt2 opt3 opt4)])
This distributes the ... over the four items inside the 'and' clause such
that each of the 'optionalN' identifiers matches all remaining elements.
'(("Name" "Age" "First" "Last")
("Name" "Age" "First" "Last")
("Name" "Age" "First" "Last")
("Name" "Age" "First" "Last"))
In hindsight it makes sense -- the 'and' causes it to match the element
across all four patterns. They all match because they are identifiers and
therefore match anything. Then the '...' causes it to do that for all
remaining elements, generating lists into each of the identifiers because
that's what '...' does.
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