id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
6040,Adding a type signature changes heap allocation into stack allocation without changing the actual type,tibbe,simonpj,"According to Milan Straka, changing

{{{
insert :: Ord k => k -> a -> Map k a -> Map k a
insert = go
  where
    STRICT_1_OF_3(go)
    go kx x Tip = singleton kx x
    go kx x (Bin sz ky y l r) = ...
}}}

to

{{{
insert :: Ord k => k -> a -> Map k a -> Map k a
insert = go
  where
    go :: Ord k => k -> a -> Map k a -> Map k a
    STRICT_1_OF_3(go)
    go kx x Tip = singleton kx x
    go kx x (Bin sz ky y l r) = ...
}}}

changes how GHC allocates the argument, from heap to stack. Here's the relevant commit: https://github.com/haskell/containers/commit/32d84ba5eb82f34dbb8a8fabf07077d848cdb408

It includes this comment:
{{{
-- [Note: Type of local 'go' function]
-- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- If the local 'go' function uses an Ord class, it must be given a type
-- which mentions this Ord class. Otherwise it is not passed as an argument and
-- it is instead heap-allocated at the entry of the outer method.
}}}

I find this quite alarming. The type of `k` above is already Ord k, so the extra type signature shouldn't make a difference in my opinion.
",bug,new,normal,7.8.1,Compiler,7.4.1,,,fox@…,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,,,,