Why does calling sumr on a stream with 50 tuples not complete

Question

While investigating a bug today  I noticed that calling sumr on a stream with 50  Int  Int  tuples never completes  but it does on a smaller stream  Calling  toList on the larger stream first completes as well    Is this the intended behavior when calling sumr on a large stream  Does it not evaluate the stream to completion  or is something else causing this   scala gt  val strSmall   Stream  1 1   2 4   3 9   4 16   5 25   strSmall  scala collection immutable Stream  Int  Int     Stream  1 1       scala gt  val strBig   Stream  1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    0 1    1 0    1 0    1 0    1 0    0 1    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    1 0    0 1    1 0    1 0    1 0    1 0    1 0   strBig  scala collection immutable Stream  Int  Int     Stream  1 0       scala gt  strSmall sumr res3   Int  Int     15 55   scala gt  strBig toList sumr res4   Int  Int     47 3   scala gt  strBig sumr  lt  -- never completes -- gt

User · Answer

sumr is implemented in terms of foldRight    final def sumr implicit A  Monoid A    A   F foldRight self  A zero  A append    foldRight is not always tail recursive  so you can overflow the stack if the collection is too long  See Why foldRight and reduceRight are NOT tail recursive  for some more discussion of when this is or isn t true

[scala] Why does calling sumr on a stream with 50 tuples not complete

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