CODEWITHSUNDEEP
f#monads
dagelee
dagelee

Reputation: 243

Explanation of Monad laws in F#

Here is Explanation of Monad laws in Haskell.

How do explain Monad laws in F#?

  1. bind (M, return) is equivalent to M.

  2. bind ((return x), f) is equivalent to f x.

  3. bind (bind (m, f),g) is equivalent to bind(m, (fun x -> bind (f x, g))).

Upvotes: 9

Views: 1083

Answers (1)

Tomas Petricek
Tomas Petricek

Reputation: 243061

I think that a good way to understand them in F# is to look at what they mean using the computation expression syntax. I'll write m for some computation builder, but you can imagine that this is async or any other computation type.

Left identity

m { let! x' = m { return x }   =   m { let x' = x
    return! f x' }                     return! f x' }

Right identity

m { let! x = comp              =   m { return! comp }
    return x }

Associativity

m { let! x = comp              =   m { let! y = m { let! x = comp
    let! y = f x                                    return! f x }
    return! g y }                      return! g y }

The laws essentially tell you that you should be able to refactor one version of the program to the other without changing the meaning - just like you can refactor ordinary F# programs.

Upvotes: 14

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