std.writer

Implementation of the Writer type

Types

type Writer w a = { value : a, writer : w }

The writer Monad

Values

#[implicit]
let functor : forall w . [Monoid w] -> Functor (Writer w)

A Functor represents an action on a parameterized type which does not change the structure with the mapped type.

The following laws should hold:

  • map id == id
  • map (f << g) == map f << map g

#[implicit]
let applicative : forall w . [Monoid w] -> Applicative (Writer w)

A Functor with application.

The following laws should hold:

  • wrap id <*> v = v
  • wrap (<<) <> u <> v <> w = u <> (v <*> w)
  • wrap f <*> wrap x = wrap (f x)
  • u <> wrap y = wrap (\g -> g x) <> u

#[implicit]
let monad : forall w . [Monoid w] -> Monad (Writer w)

A generalised interface for imperatively sequencing actions

let tell w : forall w . w -> Writer w ()