trace output statements
i have this function: -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getBD :: Connection -> String -> IO Float getBD conn name = trace "Entering getBD" noBDfp where qry_head = "select `N° BD` from Coordonnées where Nom = ?" :: Query bd_rows :: IO [Only Text] bd_rows = query conn qry_head (Only (name::String)) -- noBDtxt :: [Text] -- noBDtxt = fromOnly (Prelude.head bd_rows) -- noBDtxt :: IO [Text] -- lg = fmap Prelude.length bd_rows -- lg2 = fmap show lg noBDtxt :: IO Text -- noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows) noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . Prelude.head) bd_rows) -- noBDstr :: String -- noBDstr = Text.unpack noBDtxt noBDstr :: IO String noBDstr = trace "assigning noBDstr" (fmap Text.unpack noBDtxt) -- noBDfp = read $ noBDstr :: Float noBDfp :: IO Float noBDfp = fmap read noBDstr call by : let lstBD = Prelude.map (\elem -> getBD conn (Text.unpack (fst elem))) rows it works ok, in fact at some point it fails due to NULL sql value not again handle correctly , i have inserted trace statement that output variable, i understand it's not really good becaus it breaks the purity of haskell function, perheaps for this reason i have strange behavior of output: ... Entering getBD assigning noBDstr assigning noBDtxt Entering getBD assigning noBDstr assigning noBDtxt Entering getBD assigning noBDstr assigning noBDtxt *** Exception: UnexpectedNull {errSQLType = "VarString", errHaskellType = "Text", errFieldName = "N\194\176 BD", errMessage = "unexpected null in table Coordonn\195\169es of database sidonie"} *Main> you will notice noBDstr seems to be assigned before noBDtxt, but in the code i have the statements in this order: noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . Prelude.head) bd_rows) noBDstr :: IO String noBDstr = trace "assigning noBDstr" (fmap Text.unpack noBDtxt) i just want to understand wht's happening, this is not critical for code as it works... any idea? Damien
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation). % cat test.hs import Debug.Trace result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
lazyness....? On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
Yes, exactly! On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote:
lazyness....?
On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
i will spent part of my christmas holidays, learning seriously Haskell...fonctor,monads,IO.... i hope next year i will ask less silly question ;-) in the haskell-cafe mailing list. For now i'm sticked with problems with IO Monads,database IO, and i have to revamp all my code because it's not working as i want, having code in main is ok but if i put some part of code in a simple function it fails to compile or worse compile but ouput nothing or give/require me /back IO * things that are a nightmare to deal with... i must admit i never has such difficulty learning a language... i think learning Monads from scratch again will help me, the frustrating part of all this is that i'm only trying to do database manipulation that would be easy in python or Scheme. I do not want to blame myself more, i think that Haskell is too abstract for basic things such as IO, it's ok to be abstract for hard problems,C++ with standart template library for example but not for reading an input from a database and manipulate the data simply... On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
Yes, exactly!
On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote:
lazyness....?
On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
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I think forgetting about monads and just using do-notation will help you. On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei wrote:
i think learning Monads from scratch again will help me
On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
Yes, exactly!
On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote:
lazyness....?
On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
yes i use do notation, but for example i have code that works in main and not in a function! i print the code perheaps someone could help me: first the function, so you have the import list too: import Database.MySQL.Simple import Database.MySQL.Simple.QueryResults import Database.MySQL.Simple.Result import Database.MySQL.Simple.QueryParams import Database.MySQL.Simple.Param import Control.Monad import Data.Text as Text --import Data.Int as Int --import Data.List import Debug.Trace import Data.Maybe as Maybe -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getBD3 :: Connection -> String -> Float getBD3 conn name = do let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float return noBDfp with this function i have this error: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:54:13: error: • Couldn't match expected type ‘Float’ with actual type ‘IO Float’ • In a stmt of a 'do' block: (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) In the expression: do let qry_head_BD_Sidonie = ... (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) let noBDtxt = ... let noBDstr = ... .... In an equation for ‘getBD3’: getBD3 conn name = do let qry_head_BD_Sidonie = ... (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) let noBDtxt = ... .... | 54 | (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. i do not understand the error complaining that i return an IO float,because i'm sure it's a float in noBDfp if i put the same lines of code in the main it works !!! : main :: IO () main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie" } let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) putStr "bd_rows =" putStrLn $ show bd_rows let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float putStr "noBDfp =" (putStrLn (show noBDfp)) close conn it works i have output like this: *Main> main bd_rows =[Only {fromOnly = "-04.3982"}] noBDtxt ="-04.3982" noBDfp =-4.3982 noBDfp + 1 = -3.3982 i'm well getting a float in noBDfp , i even can add 1 to it :-) ( cool haskell...) but i'm just wanting to that in the function getDB3 but it does not compile... ?????? Damien On Sun, Dec 23, 2018 at 4:54 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
I think forgetting about monads and just using do-notation will help you.
On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei wrote:
i think learning Monads from scratch again will help me
On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
Yes, exactly!
On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote:
lazyness....?
On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
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_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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The correct type annotation for getDB3 should be: getDB3 :: Connection -> String -> IO Float Note the IO at the end. Functions in Haskell are just pure computation; they can't have side effects -- so a function returning a Float can't possibly talk to a database. Instead, The type `IO a` represents a description of an action to perform. It's still just a value -- calling getDB3 doesn't *do* anything. You can stitch these together using do-notation or functions like >>=, and when the program is run the action defined by 'main' is performed. --- An analogy: you could imagine instead of IO we could give up, and write code that computes a program in some other (imperative) programming language) that we then hand off to an interpreter. For example, we could compute a python program that counts from 1 to 99 like so: printNum :: Int -> String printNum n = "print('" ++ show n ++ "')\n" pythonProgram = concatMap printNum [1..99] So we've defined a variable fullProgram that is a string with the source code to a python program like: print('1') print('2') print('3') ... print('99') ..but we haven't actually run it. To do that we'd have to pass the string off to the python interpreter. This is a good way to think about what IO is -- main is like our pythonProgram above, in that it is a description of actions to perform, but *evaluating* it doesn't have any side effects -- it just computes the description. When you run a Haskell program, this is like taking the description defined by Main and passing it off to an interpreter. --- So your definition of getDB3 is a description of actions to perform to get a float from the database, but your type declaration says it's a function that computes a float (without having to perform any "actions"). This is a critical distinction that exists in Haskell but not most other languages. Hope this helps, -Ian Quoting Damien Mattei (2018-12-25 15:07:35)
yes i use do notation, but for example i have code that works in main and not in a function! i print the code perheaps someone could help me: first the function, so you have the import list too: import Database.MySQL.Simple import Database.MySQL.Simple.QueryResults import Database.MySQL.Simple.Result import Database.MySQL.Simple.QueryParams import Database.MySQL.Simple.Param import Control.Monad import Data.Text as Text --import Data.Int as Int --import Data.List import Debug.Trace import Data.Maybe as Maybe -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getBD3 :: Connection -> String -> Float getBD3 conn name = do � � � � � � � � � � � let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query � � � � � � � � � � � (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) � � � � � � � � � � � let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text � � � � � � � � � � � let noBDstr = Text.unpack noBDtxt :: String � � � � � � � � � � � let noBDfp = read $ noBDstr :: Float � � � � � � � � � � � return noBDfp with this function i have this error: Prelude> :load UpdateSidonie [1 of 1] Compiling Main� � � � � � � � � � � � ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:54:13: error: � � � � Couldn't match expected type �Float� with actual type �IO Float� � � � � In a stmt of a 'do' block: � � � � � � � (bd_rows :: [Only Text]) <- query � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � conn qry_head_BD_Sidonie (Only (name :: String)) � � � � � In the expression: � � � � � � � do let qry_head_BD_Sidonie = ... � � � � � � � � � � (bd_rows :: [Only Text]) <- query � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � conn qry_head_BD_Sidonie (Only (name :: String)) � � � � � � � � � � let noBDtxt = ... � � � � � � � � � � let noBDstr = ... � � � � � � � � � � .... � � � � � In an equation for �getBD3�: � � � � � � � � � getBD3 conn name � � � � � � � � � � � = do let qry_head_BD_Sidonie = ... � � � � � � � � � � � � � � � � (bd_rows :: [Only Text]) <- query � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � conn qry_head_BD_Sidonie (Only (name :: String)) � � � � � � � � � � � � � � � � let noBDtxt = ... � � � � � � � � � � � � � � � � .... � � | 54 |� � � � � � � � � � � � (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) � � |� � � � � � � � � � � � ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^ Failed, no modules loaded. i do not understand the error complaining that i return an IO float,because i'm sure it's a float in noBDfp if i put the same lines of code in the main it works !!! : main :: IO () main = � do � � � conn <- connect defaultConnectInfo � � � � � { connectHost = "moita", � � � � � � � connectUser = "mattei", � � � � � � � connectPassword = "sidonie2", � � � � � � � connectDatabase = "sidonie" } � let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query � (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) putStr "bd_rows =" putStrLn $ show bd_rows � � � let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text � � � let noBDstr = Text.unpack noBDtxt :: String � � � let noBDfp = read $ noBDstr :: Float � � � putStr "noBDfp =" � � � (putStrLn (show noBDfp)) � close conn it works i have output like this: *Main> main bd_rows =[Only {fromOnly = "-04.3982"}] noBDtxt ="-04.3982" noBDfp =-4.3982 noBDfp + 1 = -3.3982 i'm well getting a float in noBDfp , i even can add 1 to it :-) ( cool haskell...) but i'm just wanting to that in the function getDB3 but it does not compile... ?????? Damien
On Sun, Dec 23, 2018 at 4:54 PM Tom Ellis <[1]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
I think forgetting about monads and just using do-notation will help you. On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei wrote: > i think learning Monads from scratch again will help me > > On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < > [2]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote: > > > Yes, exactly! > > > > On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote: > > > lazyness....? > > > > > > On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < > > > [3]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote: > > > > > > > On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote: > > > > > i have inserted trace statement that output variable > > > > > ... i have strange behavior of output: > > > > > > > > Let's take a simpler example.� Do you understand why the trace > > statments > > > > from this small program appear in the order that they do?� (And for > > what > > > > it's worth I really think you'll be better off writing programs using > > do > > > > notation). > > > > > > > > > > > > % cat test.hs > > > > import Debug.Trace > > > > > > > > result = > > > >� � let a = trace "evaluating a" 2 > > > >� � � � b = trace "evaluating b" 10 > > > >� � � � c = trace "evaluating c" (a + b) > > > >� � in c > > > > ~% ghci -e result test.hs > > > > evaluating c > > > > evaluating b > > > > evaluating a > > > > 12 > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > [4]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > [5]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: [6]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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Tom, Why does "b" evaluate before "a" in your example? I would have thought left-hand and then right-hand if needed. I'm on my phone but I'd like to try the same example with booleans and `&&` instead of `+`. Erik On Sat, Dec 22, 2018, 11:40 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
There are no guarantees about in what order these things will be evaluated. The compiler is well within its rights to evaluate the expressions in any order, or more than once even (though IIRC ghc never does the latter). The left-to-right ordering holds for && because the Haskell report specifically defines[1] it as: True && x = x False && _ = False In this case the compiler can't in general evaluate the RHS first, because if the LHS is False and the RHS is bottom, this would be incorrect. But this is due to the semantics of &&, and doesn't hold in general. -Ian [1]: https://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 Quoting erik (2018-12-23 15:19:04)
Tom, Why does "b" evaluate before "a" in your example? I would have thought left-hand and then right-hand if needed. I'm on my phone but I'd like to try the same example with booleans and `&&` instead of `+`. Erik On Sat, Dec 22, 2018, 11:40 PM Tom Ellis <[1]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote: > i have inserted trace statement that output variable > ... i have strange behavior of output: Let's take a simpler example.� Do you understand why the trace statments from this small program appear in the order that they do?� (And for what it's worth I really think you'll be better off writing programs using do notation). % cat test.hs import Debug.Trace result = � let a = trace "evaluating a" 2 � � � b = trace "evaluating b" 10 � � � c = trace "evaluating c" (a + b) � in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: [2]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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Ah, I see. I erroneously assumed binary operators would be defined in
similar ways but I realize that's perhaps naive because with addition
you're never not going to evaluate both sides, unlike with booleans.
That's interesting. Thanks for responding.
On Sun, Dec 23, 2018, 2:26 PM Ian Denhardt There are no guarantees about in what order these things will be
evaluated. The compiler is well within its rights to evaluate
the expressions in any order, or more than once even (though IIRC
ghc never does the latter). The left-to-right ordering holds for &&
because the Haskell report specifically defines[1] it as: True && x = x
False && _ = False In this case the compiler can't in general evaluate the RHS first,
because if the LHS is False and the RHS is bottom, this would be
incorrect. But this is due to the semantics of &&, and doesn't hold in
general. -Ian [1]:
https://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 Quoting erik (2018-12-23 15:19:04) Tom,
Why does "b" evaluate before "a" in your example? I would have thought
left-hand and then right-hand if needed. I'm on my phone but I'd like
to try the same example with booleans and `&&` instead of `+`.
Erik
On Sat, Dec 22, 2018, 11:40 PM Tom Ellis
<[1]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk wrote: On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
> i have inserted trace statement that output variable
> ... i have strange behavior of output:
Let's take a simpler example.� Do you understand why the trace
statments
from this small program appear in the order that they do?� (And for
what
it's worth I really think you'll be better off writing programs
using do
notation).
% cat test.hs
import Debug.Trace
result =
� let a = trace "evaluating a" 2
� � � b = trace "evaluating b" 10
� � � c = trace "evaluating c" (a + b)
� in c
~% ghci -e result test.hs
evaluating c
evaluating b
evaluating a
12
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participants (4)
-
Damien Mattei -
erik -
Ian Denhardt -
Tom Ellis