CODEWITHSUNDEEP
haskellsyntaxfunctional-programmingpattern-matchingnon-exhaustive-patterns
Huddlespith
Huddlespith

Reputation: 123

Haskell take first and seventh

I need to write a function in Haskell that given a list of at least 7 elements returns a tuple containing the first and seventh element.

e.g. 

Prelude> take1and7 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
(1, 7)

I've tried this

take1and7 :: [a] -> (a, a)
take1and7 [a, b, c, d, e, f, g, xs] = (a, g)

But it says that it has "non-exhaustive patterns" which I don't understand

Upvotes: 0

Views: 244

Answers (1)

Will Ness
Will Ness

Reputation: 71065

... [a, b, c, d, e, f, g, xs] = ...

is a pattern which matches an 8 elements long list. xs names the 8th element, not the rest of the list.

In case you call this function with a list of 6, 7, 9, etc. elements, the pattern won't match.

... (a: b: c: d: e: f: g: xs) = ...

is a pattern which matches a list of 7 elements or longer. xs names the rest of the list after its first 7 elements. With it, the function still won't be able to handle any list shorter than 7 elements.

The "non-exhaustive" phrasing means our function doesn't handle all possible cases.

Upvotes: 6

Related Questions