Re: [Haskell-cafe] What is the rank of a polymorphic type?

[Stefan Holdermans]

Eugene,

Consider the type: (forall a . a) -> String.

It's of rank 2.

What is the definition of rank of a polymorphic type?

The minimal rank of a type is given by

  rank (forall a. t) = 1 `max` rank t
  rank (t -> u)      = (if rank t == 0 then 0 else rank t + 1) `max`  
rank u
  rank _             = 0

HTH,

  Stefan
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