Make genericLength tail-recursive so it doesn't overflow stack

A likely use for genericLength is to count lists of more than Int elements. However, the source code is not tail-recursive, making a poor "consumer", so genericLength easily overflows stack.

Here is the source code from 6.10.1:

genericLength           :: (Num i) => [b] -> i
genericLength []        =  0
genericLength (_:l)     =  1 + genericLength l

Here is a proposed alternative:

genericLength ∷ (Num i) ⇒ [b] → i
genericLength = len 0 where
  len n [] = n
  len n (_:xt) = len (n+1) xt

In my test application (enumerating the 66,960,965,307 atomic lattices on six atoms) this alternative avoids overflowing the stack.

[This is not the same issue as http://hackage.haskell.org/trac/ghc/ticket/2962]

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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