[Haskell-cafe] working through "Part I: Dependent Types in Haskell"

Andres Löh andres at well-typed.com
Mon Apr 27 12:31:28 UTC 2015


> init' :: Vector a ('S n) -> Vector a n
> init' (x1 :- Nil) = Nil
> init' (x :- xs) = x :- (init' xs)

You need to pattern match further on `xs` in order to make clear that
you require it to not be `Nil`. The type checker doesn't look at the
first case in order to conclude that you've already ruled that out.

So e.g.

    init' (x :- xs @ (_ :- _)) = x :- (init' xs)



Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com

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