A data type which contains itself. The commonest example is the list type, in ref="module.php?name=Lexikon&file=search&eid=1&query=Haskell">Haskell: data List a = Nil | Cons a (List a) which says a list of a' s is either an empty list or a {cons cell} containing an ' a' (the "head" of the list) and another list (the "tail"). Recursion is not allowed in ref="module.php?name=Lexikon&file=search&eid=1&query=Miranda">Miranda or Haskell {synonym type}s, so the following {Haskell} types are illegal: type Bad = (Int, Bad) type Evil = Bool -> Evil whereas the seeminly equivalent ref="module.php?name=Lexikon&file=search&eid=1&query=algebraic data type">algebraic data types are acceptable: data Good = Pair Int Good data Fine = Fun (Bool->Fine)

In addition suitable contents:
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ref="module.php?name=Lexikon&op=content&tid=1708">C ] [ ref="module.php?name=Lexikon&op=content&tid=1896">cc ] [ ref="module.php?name=Lexikon&op=content&tid=1944">cell ] [ ref="module.php?name=Lexikon&op=content&tid=2001">ch ] [ ref="module.php?name=Lexikon&op=content&tid=2247">co ] [ ref="module.php?name=Lexikon&op=content&tid=2330">com ] [ ref="module.php?name=Lexikon&op=content&tid=2545">con ] [ ref="module.php?name=Lexikon&op=content&tid=2606">cons ] [ ref="module.php?name=Lexikon&op=content&tid=2900">cu ] [ ref="module.php?name=Lexikon&op=content&tid=3006">data ] [ ref="module.php?name=Lexikon&op=content&tid=3091">data type ] [ ref="module.php?name=Lexikon&op=content&tid=3752">du ] [ ref="module.php?name=Lexikon&op=content&tid=3834">E ] [ ref="module.php?name=Lexikon&op=content&tid=3865">ec ] [ ref="module.php?name=Lexikon&op=content&tid=3896">ed ] [ ref="module.php?name=Lexikon&op=content&tid=3929">ee ] [ ref="module.php?name=Lexikon&op=content&tid=3946">eg ] [ 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ref="module.php?name=Lexikon&op=content&tid=6482">is ] [ ref="module.php?name=Lexikon&op=content&tid=6558">it ] [ ref="module.php?name=Lexikon&op=content&tid=6789">ke ] [ ref="module.php?name=Lexikon&op=content&tid=7068">legal ] [ ref="module.php?name=Lexikon&op=content&tid=7091">Lex ] [ ref="module.php?name=Lexikon&op=content&tid=7107">li ] [ ref="module.php?name=Lexikon&op=content&tid=7220">list ] [ ref="module.php?name=Lexikon&op=content&tid=7441">ly ] [ ref="module.php?name=Lexikon&op=content&tid=7457">M ] [ ref="module.php?name=Lexikon&op=content&tid=7979">Miranda ] [ ref="module.php?name=Lexikon&op=content&tid=8019">mm ] [ ref="module.php?name=Lexikon&op=content&tid=8032">mo ] [ ref="module.php?name=Lexikon&op=content&tid=8040">mod ] [ ref="module.php?name=Lexikon&op=content&tid=8079">module ] [ ref="module.php?name=Lexikon&op=content&tid=8167">mp ] [ ref="module.php?name=Lexikon&op=content&tid=8384">N ] [ ref="module.php?name=Lexikon&op=content&tid=8386">na ] [ 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ref="module.php?name=Lexikon&op=content&tid=11150">se ] [ ref="module.php?name=Lexikon&op=content&tid=11376">si ] [ ref="module.php?name=Lexikon&op=content&tid=11510">sk ] [ ref="module.php?name=Lexikon&op=content&tid=11651">so ] [ ref="module.php?name=Lexikon&op=content&tid=11934">st ] [ ref="module.php?name=Lexikon&op=content&tid=12246">sy ] [ ref="module.php?name=Lexikon&op=content&tid=12359">T ] [ ref="module.php?name=Lexikon&op=content&tid=12369">table ] [ ref="module.php?name=Lexikon&op=content&tid=12588">th ] [ ref="module.php?name=Lexikon&op=content&tid=12970">type ] [ ref="module.php?name=Lexikon&op=content&tid=13252">va ] [ ref="module.php?name=Lexikon&op=content&tid=13366">vi ] [ ref="module.php?name=Lexikon&op=content&tid=13725">win ]