"http://moonbase.rydia.net/mental/writings/programming/monads-in-ruby/"

"Serially-numbering monad in Slate:"

"Defining a datatype for Integer-tagged values..."
define: #IntegerTagging &slots: #(tag value).

obj tagWith: i@(Integer traits)
[IntegerTagging clone `>> [tag: i. value: obj. ]].

"Defining a counting monad with its two operations (global)..."
_@here return: val
[[| :currentCounter | currentCounter tagWith: val]].

_@here bind: monad to: m@(Method traits)
[
  [| :currentCounter result newM |
   result: (monad applyWith: currentCounter).
   newM: (m applyWith: result value).
   newM applyWith: result tag]
].

"Verification:"

"return: is the left and right unit of bind:to: ..."
(bind: (return: v) to: m) === (m applyWith: v)
(bind: monad to: [| :v | return: v]) === monad

"bind:to: is associative..."
(bind: monad to: [| :v | bind: (f applyWith: v) to: g])
  === (bind: (bind: monad to: f) to: g)

"Defining an increment morphism..."

_@here incr [[| :n | n tagWith: n + 1]].

"An operation to run the monad..."

_@here runMonad: monad with: initCounter
[monad applyWith: initCounter].

"Example..."

define: #Tree &slots: {#value. #children -> {}}.

"Starting over..."

define: #Monad &parents: {Cloneable} &slots: {#value #action}.

m@(Monad traits) newFor: obj
[m clone `>> [value: obj. ]].

m@(Monad traits) newDoing: action
[m clone `>> [action: action. ]].

m@(Monad traits) pass
[m action applyWith: m value].

define: #IdentityMonad -> (Monad newDoing: Method Identity).