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+! Copyright (C) 2008 Slava Pestov
+! See http://factorcode.org/license.txt for BSD license.
+USING: arrays kernel sequences sequences.deep splitting
+accessors fry locals combinators namespaces lazy-lists
+shuffle ;
+IN: monads
+
+! Functors
+GENERIC# fmap 1 ( functor quot -- functor' ) inline
+
+! Monads
+
+! Mixin type for monad singleton classes, used for return/fail only
+MIXIN: monad
+
+GENERIC: monad-of ( mvalue -- singleton )
+GENERIC: return ( string singleton -- mvalue )
+GENERIC: fail ( value singleton -- mvalue )
+GENERIC: >>= ( mvalue -- quot )
+
+M: monad return monad-of return ;
+M: monad fail   monad-of fail   ;
+
+: bind ( mvalue quot -- mvalue' ) swap >>= call ;
+: >>   ( mvalue k -- mvalue' ) '[ drop , ] bind ;
+
+:: lift-m2 ( m1 m2 f monad -- m3 )
+    m1 [| x1 | m2 [| x2 | x1 x2 f monad return ] bind ] bind ;
+
+:: apply ( mvalue mquot monad -- result )
+    mvalue [| value |
+        mquot [| quot |
+            value quot call monad return
+        ] bind
+    ] bind ;
+
+M: monad fmap over '[ @ , return ] bind ;
+
+! 'do' notation
+: do ( quots -- result ) unclip dip [ bind ] each ;
+
+! Identity
+SINGLETON: identity-monad
+INSTANCE:  identity-monad monad
+
+TUPLE: identity value ;
+INSTANCE: identity monad
+
+M: identity monad-of drop identity-monad ;
+
+M: identity-monad return drop identity boa ;
+M: identity-monad fail   "Fail" throw ;
+
+M: identity >>= value>> '[ , _ call ] ;
+
+: run-identity ( identity -- value ) value>> ;
+
+! Maybe
+SINGLETON: maybe-monad
+INSTANCE:  maybe-monad monad
+
+SINGLETON: nothing
+
+TUPLE: just value ;
+: just \ just boa ;
+
+UNION: maybe just nothing ;
+INSTANCE: maybe monad
+
+M: maybe monad-of drop maybe-monad ;
+
+M: maybe-monad return drop just ;
+M: maybe-monad fail   2drop nothing ;
+
+M: nothing >>= '[ drop , ] ;
+M: just    >>= value>> '[ , _ call ] ;
+
+: if-maybe ( maybe just-quot nothing-quot -- )
+    pick nothing? [ 2nip call ] [ drop [ value>> ] dip call ] if ; inline
+
+! Either
+SINGLETON: either-monad
+INSTANCE:  either-monad monad
+
+TUPLE: left value ;
+: left \ left boa ;
+
+TUPLE: right value ;
+: right \ right boa ;
+
+UNION: either left right ;
+INSTANCE: either monad
+
+M: either monad-of drop either-monad ;
+
+M: either-monad return  drop right ;
+M: either-monad fail    drop left ;
+
+M: left  >>= '[ drop , ] ;
+M: right >>= value>> '[ , _ call ] ;
+
+: if-either ( value left-quot right-quot -- )
+    [ [ value>> ] [ left? ] bi ] 2dip if ; inline
+
+! Arrays
+SINGLETON: array-monad
+INSTANCE:  array-monad monad
+INSTANCE:  array monad
+
+M: array-monad return  drop 1array ;
+M: array-monad fail   2drop { } ;
+
+M: array monad-of drop array-monad ;
+
+M: array >>= '[ , _ map concat ] ;
+
+! List
+SINGLETON: list-monad
+INSTANCE:  list-monad monad
+INSTANCE:  list monad
+
+M: list-monad return drop 1list ;
+M: list-monad fail   2drop nil ;
+
+M: list monad-of drop list-monad ;
+
+M: list >>= '[ , _ lmap lconcat ] ;
+
+! State
+SINGLETON: state-monad
+INSTANCE:  state-monad monad
+
+TUPLE: state quot ;
+: state \ state boa ;
+
+INSTANCE: state monad
+
+M: state monad-of drop state-monad ;
+
+M: state-monad return drop '[ , 2array ] state ;
+M: state-monad fail   "Fail" throw ;
+
+: mcall quot>> call ;
+
+M: state >>= '[ , _ '[ , mcall first2 @ mcall ] state ] ;
+
+: get-st ( -- state ) [ dup 2array ] state ;
+: put-st ( value -- state ) '[ drop , f 2array ] state ;
+
+: run-st ( state initial -- ) swap mcall second ;
+
+: return-st state-monad return ;
+
+! Reader
+SINGLETON: reader-monad
+INSTANCE:  reader-monad monad
+
+TUPLE: reader quot ;
+: reader \ reader boa ;
+INSTANCE: reader monad
+
+M: reader monad-of drop reader-monad ;
+
+M: reader-monad return drop '[ drop , ] reader ;
+M: reader-monad fail   "Fail" throw ;
+
+M: reader >>= '[ , _ '[ dup , mcall @ mcall ] reader ] ;
+
+: run-reader ( reader env -- ) swap mcall ;
+
+: ask ( -- reader ) [ ] reader ;
+: local ( reader quot -- reader' ) swap '[ @ , mcall ] reader ;
+
+! Writer
+SINGLETON: writer-monad
+INSTANCE:  writer-monad monad
+
+TUPLE: writer value log ;
+: writer \ writer boa ;
+
+M: writer monad-of drop writer-monad ;
+
+M: writer-monad return drop { } writer ;
+M: writer-monad fail   "Fail" throw ;
+
+: run-writer ( writer -- value log ) [ value>> ] [ log>> ] bi ;
+
+M: writer >>= '[ , run-writer _ '[ @ run-writer ] dip append writer ] ;
+
+: pass ( writer -- writer' ) run-writer [ first2 ] dip swap call writer ;
+: listen ( writer -- writer' ) run-writer [ 2array ] keep writer ;
+: tell ( seq -- writer ) f swap writer ;