If you need the index of an element in a list you're not thinking in Haskell yet. Haskell lists are more like streams of elements. You wouldn't ask C++ for the index of an element in stdin would you? You probably want a finite length data structure such as Data.Vector.
– Jason Dagit
Does your question also apply to the more general case of the list being, say, [1,2,3,4,3]? That is, would your desired function return both of those indexes, or maybe just the first one it found?
– wlangstroth
@JasonDagit I don't think it is true. E.g. I'm doing an implementation of a database (part of a student project), and there is a header of a table with column names, and a list of rows with according values. Given a key, the only way I see, is to find an index in header, and take according to the index values from every row.
– Hi-Angel
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[1,2,3,4,3]? That is, would your desired function return both of those indexes, or maybe just the first one it found? – wlangstroth