Haddock numbered list continuation

You can’t. You’re using >>>. In order to have this rendered as an example, it needs to be at the beginning of the paragraph. What’s considered a beginning of the paragraph? Anything at the start of a Haddock comment, skipping any white space preceding it. Anything following an empty line. In your scenario you have … Read more

Can you define `Comonads` based on `Monads`?

Yes, in fact any functor gives rise to a unique comonad in this way, unless f==0. Let F be an endofunctor on Hask. Let W(a) = ∀r.F(a->r)->r W(f) = F(f∗)∗ where g∗(h) = h∘g The puzzle becomes geometric/combinatoric in nature once you realize the following isomorphism: Theorem 1. Suppose neither of the types (∀r.r->F(r)) (∀r.F(r)->r) … Read more

haskell regex substitution

I don’t think “just roll your own” is a reasonable answer to people trying to get actual work done, in an area where every other modern language has a trivial way to do this. Including Scheme. So here’s some actual resources; my code is from a project where I was trying to replace “qql foo … Read more

What exactly does “deriving Functor” do?

To use deriving Functor you must enable the DeriveFunctor language pragma and apply it to a polymorphic type which has a covariant final type variable—in other words, a type which admits a valid Functor instance. It’ll then derive the “obvious” Functor instance. There’s been some concern in the past that the derived instance is not … Read more

Or-patterns in Haskell

I don’t think this is possible in haskell. There are however, a few alternatives: Factor out the common code with a where binding This doesn’t make much sense in your example, but is useful if you have more code in the body of the case expression: combine o1 o2 = case (o1,o2) of (Valid, Invalid) … Read more

What is the difference between Fix, Mu and Nu in Ed Kmett’s recursion scheme package

Mu represents a recursive type as its fold, and Nu represents it as its unfold. In Haskell, these are isomorphic, and are different ways to represent the same type. If you pretend that Haskell doesn’t have arbitrary recursion, the distinction between these types becomes more interesting: Mu f is the least (initial) fixed point of … Read more

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