doc/guide/src/EtaExpansion.adoc

   1 EtaExpansion
   2 ============
   3
   4 Eta expansion is a simple syntactic change used to work around the
   5 <:ValueRestriction:> in <:StandardML:Standard ML>.
   6
   7 The eta expansion of an expression `e` is the expression
   8 `fn z => e z`, where `z` does not occur in `e`.  This only
   9 makes sense if `e` denotes a function, i.e. is of arrow type.  Eta
  10 expansion delays the evaluation of `e` until the function is
  11 applied, and will re-evaluate `e` each time the function is
  12 applied.
  13
  14 The name "eta expansion" comes from the eta-conversion rule of the
  15 <:LambdaCalculus:lambda calculus>.  Expansion refers to the
  16 directionality of the equivalence being used, namely taking `e` to
  17 `fn z => e z` rather than `fn z => e z` to `e` (eta
  18 contraction).