Draft Hat Tutorial: Part 1
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1. Here the number of observations is small Larger collections of obervations are presented in blocks of ten by default hat observe gt lt 1 a lt m True 2 r lt a False 3 g lt a False 4 o lt a False 5 p lt a False 6 r lt m False 7 g lt m True 8 o lt g False 9 r lt g False 10 p lt g False more gt Keying return in response to more gt requests the next block of observations Alternatively requests in the colon command family can be given Any other line of input cuts short the list of reported observations in favour of a fresh hat observe gt prompt Observing restricted patterns of applications Viewing a block at a time is not the only way of handling what may be a large number of applications Obervations can also be restricted to applications in which specific patterns of values occur as arguments or result or to applications in a specific context The full syntax for obervation queries is identifier pattern pattern in identifier where the indicates that there can be zero or more occurrences of an argument pattern and the indicate that the result pattern and context are optional Patterns in observation queries are simplified versions of constructor patterns with _ as the only variable Some examples for the Insort computation ha2. BadInsort hs line 7 col 19 lt putStrLn aaaaaaaall lt insert p a _ if False Notice two further features of expression display 7When non termination is suspected interrupt as quickly as possible to avoid working with very large traces 8However the trace from point of interruption depends on the timing of the interrupt e the symbol E in the string argument to putStrLn indicates the lower reaches of a large structure here the tail end of a long string that has been pruned from the display e the symbol _ in the list argument to insert indicates an expression that was never evaluated The parent application insert p a _ if False gives several impor tant clues It tells us that in the else branch of the recursive case in the definition of insert the argument s head here a is duplicated endlessly to generate the result without ever demanding the argument s tail shown only as _ This should be enough explanation to discover the fault if we didn t already know it Using hat observe Once again let s also see what happens if we use hat observe hat observe BadInsort hat observe gt info lt insert main putStrLn sort All the expected items are listed as observable We know well enough from the overall output what main and putStrLn are doing but what about sort hat observe gt sort 1 sort program a a i a d ae d ae d
3. ae d aea d ae l a ll 2 sort rogram a _ 3 sort ogram a _ 4 sort gram a _ 5 sort ram a _ 6 sort om tel 7 sort m m 8 sort 0 Observations 1 to 5 tell a similar story to hat trail the tails of the recursively computed lists are never demanded at the outermost level the head is repeated endlessly Observation 6 points to a problem other than non termination but we shall ignore that for now Observations 7 and 8 do not point to a problem at all There is one further clue in these observations there are only eight of them and the arguments decrease just as expected This suggests that the recursive loop is elsewhere in insert perhaps hat observe gt insert 1 insert p Ca _ Va VPravrarvravravavava a unl 2 insert r a _ a _ 3 insert o a _ a _ 4 insert g a _ a _ 5 insert a m a 6 insert m m searching C to interrupt Interrupted 10 There is a distinct pause after observation 6 Many more observations would eventually be reported because hat observe lists each observation that is distinct from or more general than those listed previously When the computation is interrupted there are many different applications of the form insert p a _ in progress each with results evaluated to a different extent But observation 1 is enough As the tail of t
4. of the current selection backspace remove the last addition to the trail display arrow keys select a parts of the output generated by different actions or b subexpressions of expressions already on display And a basic repertoire of command line requests is source show the source expression of which the current selection is an instance quit finish this hat trail session It is enough to give initial letters s or q rather than source or quit Some Insort trails To trace the output from the Insort computation keying return alters the Trail part of the display to Trails e site Insort hs line 10 col 8 lt putStrLn agmoprr The source reference is to the corresponding application of putStrLn in the program Giving the command s at this point creates a separate source window showing the relevant extract of the program Back to the Trail display Keying return again Trall Asaan saa sas ars Insort hs line 10 col 1 lt putStrLn agmoprr lt main That is the line of output was produced by an application of putStrLn occurring in the body of main So far so good but what about the sorting How do we see where putStr s string argument agmoprr came from By making that string the current selection and requesting its parent 4In the printed version of this tutorial highlighted text or expressions are shown boxed the Hat tools actually use colour f
5. showing for each application of the function to distinct arguments what result was computed The user has the option to limit observations to particular patterns of arguments or results or to particular application contexts Hat observe sessions and requests A hat observe session can be started from a shell command line or from within existing sessions of hat tools hat observe Insort hat observe gt In comparison with hat trail there is more emphasis on command lines in hat observe and the main user interface is a prompt request response cycle Re quests are of two kinds Some are observation queries in the form of application patterns the simplest observation query is just the name of a top level function or defined value Others are command lines starting with a colon similar to those of hat trail A basic repertoire of command line requests is info list the names of functions and other defined values that can be observed quit finish this hat observe session Again it is enough to give the initial letters i or q Some Insort observations A common way to begin a hat observe session is with an info request followed by initial observation of central functions hat observe gt info lt insert main putStrLn sort hat observe gt sort 1 sort program agmoprr 2 sort rogram agmorr 3 sort ogram agmor 4 sort gram agmr 5 sort ram amr 6 sort om om 7 sort m m 8 sort
6. Draft Hat Tutorial Part 1 Colin Runciman June 13 2002 1 Introduction This tutorial is intended as a practical introduction to the Hat tools for tracing Haskell 98 programs It introduces the basic ideas and explains with worked examples how to use the tools Readers are encouraged to follow the tutorial using an installation of Hat This first version of the tutorial assumes hat Version 2 00 nhc98 Version 1 14 hmake Version 3 05 and Linux but it works equally well with ghc instead of nhc98 There are several Hat tools for examining traces but the tutorial will con sider only the two used most hat trail and hat observe Even for these tools not every option and command is discussed For a more comprehensive reference see the Hat User Manual The tutorial makes use of a small example program at first a correctly working version later one with faults deliberately introduced The intended behaviour of the program is very simple it should sort the letters of the word program using insertion sort The working program is given in Figure 1 2 Hat Compilation and Execution To use Hat the Haskell program to be traced must first be compiled with the hat option to hmake hmake hat Insort hat trans Insort hs Wrote TInsort hs nhc98 package hat c o TInsort o TInsort hs nhc98 package hat o Insort TInsort o A program compiled for tracing can be executed just as if it had been com piled normally Inso
7. bservations of lt How can that be What is happening to ordered insertion hat observe gt insert insert p insert Ir insert o 2 2 insert g insert a insert m Oon BRUNE The symbol _ _ here indicates an undefined value Reading the character ar guments vertically program seems to be misspelt is there an observation missing between 4 and 5 There are in fact two separate applications insert r _ _ _ _ but duplicate observations are not listed by default The insert observations explain why there are no lt applications In all the observed applications the list arguments are undefined So neither of the defining equations for insert is ever matched and as the lt comparison is on the right hand side of the recursive equation it is never reached Why are the insert arguments undefined They should be the results of sort applications hat observe gt sort 1 sort program _ _ 2 sort rogram _ _ 3 sort ogram _ _ 4 sort gram _ _ 5 sort ram 6 T 8 sort am _ _ sort m sort With the exception of the last observation these _ _ results are exactly the results obtained from insert as already observed But hat observe unlike hat trail is not concerned with such relationships In short the story so far from hat observe is quite simple everything is undefined What about the other two items in the info list putStrLn and main hat observe gt
8. he argument is unevaluated the result would be the same whatever the tail It could be so we know insert gt p a aaaa This specific and simple failing case directs us to the fault in the definition of insert 5 3 Tracing Wrong Output Let s now correct the recursive call from insert x y ys to insert x ys recompile then execute BadInsort agop Using hat observe Once again we could reach first for hat trail to trace the fault but the availabil ity of a well defined but wrong result also suggests a possible starting point in hat observe hat observe BadInsort hat observe gt insert _ _ agop 1 insert p agor agop Somehow insertion loses the final element r Perhaps we d like to see more details of how this result is obtained the relevant recursive calls for example hat observe gt insert p _ in insert 1 insert p gor gop 2 insert p or op 3 insert 2p ae p Observation 3 makes it easy to discover the fault by inspection Using hat trail If we instead use hat trail the same application could be reached as follows We first request the parent of the output unsurprisingly it is putStrLn agop We then request the parent of the string argument agop OUD PUES SH SS SSeS Sao Sasser a SSS n ese agop n Trail BadInsort hs line 10 col 26 9 Application A is more general than application B if
9. issing from result then x ys FAULT 3 recursive call is same else y insert x y ys main putStrLn sort program Figure 2 A faulty version of the insertion sort program 5 1 Tracing a Run time Error We compile the faulty program for tracing then run it hmake hat BadInsort BadInsort No match in pattern Two questions prompted by this error message are e What was the application that didn t match e Where did that application come from Using hat trail Both questions can be answered by using hat trail to trace the derivation of the error hat trail BadInsort Erront jos sss ssss aS eS ase ta sar sss aa asa Seas s seco or essssSs No match in pattern Keying return to see the application that spawned the error Trail s 4 4 ss 4 Ss5 BadInsort hs line 3 col 25 lt sort The source command shows the site of the offending application to be the recursive call in sort If necessary the ancestry of the argument or the sort application could be traced back further for example selecting the argument right arrow twice shows its origin to be the string literal program in main Using hat observe Although hat trail is usually the first resort for tracing run time errors it is instructive to see what happens if instead we try using hat observe hat observe BadInsort hat observe gt info insert main putStrLn sort There are no o
10. or highlighting 5The only thing to do with a source extract is to look at it tracing with Hat does not involve annotating or otherwise modifying program sources removes main selects putStrLn selects agmoprr requests parent expression backspace right arrow right arrow return SS Trail s4 254 45 544 Insort hs line 7 col 19 lt putStrLn agmoprr lt insert p agmorr if False The string agmoprr is the result of inserting p the head of the string program into the recursively sorted tail More specifically the string was computed in the else branch of the conditional by which insert is defined in the recursive case because p lt a is False And so we could continue For example following the trail of string argu ments lt insert p agmorr if False lt insert r agmor if False lt insert o agmr if False lt insert g amr if False lt insert r am if False lt insert a m if True lt insert m But let s leave hat trail for now quit 4 Hat observe Basics The idea of hat observe is to answer the question How was that applied and with what results mainly in relation to a top level function Answers take the form of a list of equational observations
11. putStrLn 1 putStrLn _ _ IO putStrLn _ _ hat observe gt main 1 main IO putStrLn _ _ These observations at least confirm that the program does compute an I O action but the output string is undefined 5 2 Tracing a Non terminating Computation Suppose we correct the first fault by restoring the equation sort and recompile Now the result of running BadInsort is a non terminating computation with an infinite string aaaaaaa as output It seems that BadInsort has entered an infinite loop The computation can be interrupted by keying control C BadInsort Program interrupted C aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Questions this time include e What parts of the program does the infinite loop involve e How did it come about in the first place Using hat trail The initial hat trail display is BELO Seo ee ee ee ee ee Program interrupted C Output aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa We have a choice we can follow the trail back either from the point of interrup tion the initial selection or from the output reached by down arrow In this case it makes little difference either way we end up examining the endless list of a s Let s select the output Output aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Trail
12. rt agmoprr l Available from http www cs york ac uk fp hat 2The program may also be found in the file Insort hs sort Ord a gt a gt a sort 0 sort x xs insert x sort xs insert Ord a gt a gt a gt a insert x x insert x y ys if x lt y thenx y ys else y insert x ys main putStrLn sort program Figure 1 An insertion sort program The main difference from untraced execution is that as Insort runs it records a detailed trace of its computation in a file Insort hat The trace is a graph of program expressions encoded in a special purpose binary format Two further files Insort hat output and Insort hat bridge record the output and associated references to the trace file 3 Hat trail Basics After a program compiled for tracing has been run creating a trace file special purpose tools are used to examine the trace The first such tool we shall look at is hat trail The idea of hat trail is to answer the question Where did that come from in relation to values expressions outputs and error messages The immediate answer will be a parent application or name More specifically e errors the application or name being reduced when the error occurred eg head might be the parent of a pattern match failure e outputs the monadic action that caused the output eg putStr Hello world might the parent of a section of output text e non val
13. t observe gt insert g 1 insert g amr agmr 2 insert g mr gmr hat observe gt insert _ _ _ 1 insert m m 2 insert r r hat observe gt sort in main 1 sort program agmoprr hat observe gt sort in sort 1 sort rogram agmorr 2 sort ogram agmor 3 sort gram agmr 4 sort ram amr 5 sort am am 6 sort m m 7 sort Enough on hat observe for now hat observe gt quit 5 Tracing Faulty Programs We have seen so far some of the ways in which Hat tools can be used to trace a correctly working program But a common and intended use for Hat is to trace a faulty program with the aim of discovering and understanding its faults A faulty computation has one of three outcomes 1 termination with a run time error or 2 termination with incorrect output or 3 non termination A variant of Insort given in Figure 2 has three faults each of which alone would cause a different outcome as indicated by comments In the following sections we shall apply the Hat tools to examine the faulty program as if we didn t know in advance where the faults were 6The program may also be found in the file BadInsort hs sort Orda gt a gt a FAULT 1 missing equation for argument sort x xs insert x sort xs insert Ord a gt a gt a gt a insert x x insert x y ys if x lt y FAULT 2 y m
14. their arguments and results agree where both are evaluated but A s arguments are less evaluated than B s or A s result is more evaluated than B s 11 lt putStrLn agop lt insert p agor if False As in hat observe we notice the loss of r in agop Back spacing over this application of insert we instead select the one character tail of agop Trail BadInsort3 hs line 9 col 26 lt putStrLn agoj p lt insert p r if True To be continued 12
15. ue expressions the application or name whose defining body con tains the expression of which the child is an instance eg append 1 2 3 4 might be the parent of append 2 3 4 e values as for non value expressions or the application of a predefined function with the child as result eg 1 2 3 4 might be the parent of 1 2 3 4 Parent expressions and their subexpressions may in turn have parents of their own The tool is called hat trail because it displays trails of ancestral redexes tracing effects back to their causes Hat trail sessions and requests A hat trail session can be started from a shell command line or from within existing sessions of hat tools The immediate result of the shell command 3Trace files do not include program sources but they do include references to program sources modifying source files could invalidate source links from traces hat trail Insort is the display of a terminal window with an upper part headed Output and a lower part headed Trail Qut put SSSsSs SSS SS Sas SSS sa Sa sos Sess eases sasse agmoprr n The line of output is highlighted because it is the current selection Requests in hat trail are of two kinds Some are single key presses with an immediate response others are command lines starting with a colon and only acted upon when completed by keying return A basic repertoire of single key requests is return add to the trail the parent expression
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