Take a look at these examples: http://rosettacode.org/wiki/Y_combinator#Haskell http://rosettacode.org/wiki/Y_combinator#OCaml http://rosettacode.o

Take a look at these examples: http://rosettacode.org/wiki/Y_combinator#Haskell http://rosettacode.org/wiki/Y_combinator#OCaml http://rosettacode.org/wiki/Y_combinator#Scala http://rosettacode.org/wiki/Y_combinator#Standard_ML http://rosettacode.org/wiki/Y_combinator#Swift The Mu/Roll trick seems to be a default way of typing Y, while some languages (like OCaml) provide other type-system features like recursive types or polymorphic variants.