Peter Hercek wrote:
Hi,
I expected ":trace expr" to always add data to the trace history but it does not do so for CAFs (which are not reduced yet). My point is that the command ":trace z" did not add anything to the trace history and I cannot check why value z is 2, because value of y is not in the trace history. Is this the expected behavior? If it is, how can I make ghci to extend the trace history when "forcing" variables?
Peter.
Here is the example:
status:0 peter@metod [765] ~/tmp % cat a.hs test :: Int -> Int test x = let y = x+1 in let z = y+1 in z status:0 peter@metod [766] ~/tmp % ghci a.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Main ( a.hs, interpreted ) Ok, modules loaded: Main. *Main> :break Main 5 Breakpoint 0 activated at a.hs:5:2 *Main> :trace test 0 Stopped at a.hs:5:2 _result :: Int = _ z :: Int = _ 4 let z = y+1 in 5 z 6 [a.hs:5:2] *Main> :back Logged breakpoint at a.hs:(2,0)-(5,2) _result :: Int 1 test :: Int -> Int 2 test x = 3 let y = x+1 in 4 let z = y+1 in 5 z 6 [-1: a.hs:(2,0)-(5,2)] *Main> :back no more logged breakpoints
ok so far - y and z have not been evaluated.
[-1: a.hs:(2,0)-(5,2)] *Main> :forward Stopped at a.hs:5:2 _result :: Int z :: Int 4 let z = y+1 in 5 z 6 [a.hs:5:2] *Main> :trace z 2
this evaluates z.
[a.hs:5:2] *Main> :back Logged breakpoint at a.hs:(2,0)-(5,2) _result :: Int 1 test :: Int -> Int 2 test x = 3 let y = x+1 in 4 let z = y+1 in 5 z 6
You are going back in the original context, but I presume you were expecting to go back in the evaluation of z. You can only go back in the context of the current evaluation, however. Try this: *Main> :break 3 Breakpoint 4 activated at trace.hs:3:10-12 *Main> :trace test 0 Stopped at trace.hs:3:10-12 _result :: Int = _ x :: Int = 90 2 test x = 3 let y = x+1 in 4 let z = y+1 in [trace.hs:3:10-12] *Main> :history -1 : test (trace.hs:4:10-12) -2 : test (trace.hs:5:2) -3 : test (trace.hs:(2,0)-(5,2)) <end of history> [trace.hs:3:10-12] *Main> :back Logged breakpoint at trace.hs:4:10-12 _result :: Int y :: Int 3 let y = x+1 in 4 let z = y+1 in 5 z [-1: trace.hs:4:10-12] *Main> :back Logged breakpoint at trace.hs:5:2 _result :: Int z :: Int 4 let z = y+1 in 5 z 6 Cheers, Simon