Add a more complete example for the special SPEC argument to the user guide

In the section for -fspec-constr in the user guide there is an example of how to use SPEC to force a loop to be unrolled by spec constr.

However, this argument doesn't do anything until foldl is called with a specific argument. It wasn't clear to me exactly how this worked as the example was incomplete. Below is a complete file which shows how the optimisation unrolls the list completely such that res2 = 6 in core.

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE ExistentialQuantification #-}
module Foo where
import GHC.Types

data Stream a = forall s.
                Stream !(s -> Step a s)  -- a stepper function
                !s                       -- an initial state

data Step a s = Yield a !s
               | Skip    !s
               | Done

s2 = Stream s [1,2,3]

s [] = Done
s (x:xs) = Yield x xs

res2 = Foo.foldl (+) 0 s2


foldl :: (a -> b -> a) -> a -> Stream b -> a
{-# INLINE foldl #-}
foldl f z (Stream step s) = foldl_loop SPEC z s
    where
      foldl_loop !sPEC z s = case step s of
                              Yield x s' -> foldl_loop sPEC (f z x) s'
                              Skip  s'     -> foldl_loop sPEC z s'
                              Done -> z

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture