Sven started to write a chapter GoodPractice on PharoForTheEntreprise.
If you want to imporve it just go.
https://github.com/SquareBracketAssociates/PharoForTheEnterprise-english/tree/master/GoodPractices
Stef
!Some Good Coding Practices
Best Smalltalk Practices by Kent Beck, Smalltalk by Example and
Smalltalk with Style are three excellent books about coding practices.
Every Smalltalkers should read them. Still there are some coding
practices that programmers should be aware of.
In this chapter we present some simple and good practices that make your
code often more efficient and avoid generating unnecessary garbage.
!!Avoid unnecessary string concatenations
In Smalltalk, the message ==,== \mthindex{String}{,} concatenates two
strings. It is handy but this message is really costly since it copies
the receiver. Therefore avoid it as much as possible and especially in
loop since it multiplies the effect.
Prefer to use \ct{streamContents:} \mthindex{String}{streamContents:},
==nextPut:== and ==nextPutAll:== since they avoid the duplication. The
message ==streamContents:== expects a block would argument is a stream
on the receiver.
Let us have a look at the following code snippet.
[[[
String streamContents: [:s |
#('Pharo''s' 'goal' 'is' 'to' 'deliver' 'a' 'clean,' 'innovative,'
'free' 'open-source' 'Smalltalk' 'environment.' 'By' 'providing' 'a'
'stable' 'and' 'small' 'core' 'system,' 'excellent' 'dev' 'tools,' 'and'
'maintained' 'releases,' 'Pharo' 'is' 'an' 'attractive' 'platform' 'to'
'build' 'and' 'deploy' 'mission' 'critical' 'Smalltalk' 'applications.')
do: [:each | s nextPutAll: each; nextPut: Character space].
s contents]
]]]
Note that we took an example with many strings to stress the effect, but
the bench results shows already a 4 factor.
[[[
[String streamContents: [:s |
#('Pharo''s' 'goal' 'is' 'to' 'deliver' 'a' 'clean,' 'innovative,'
'free' 'open-source' 'Smalltalk' 'environment.' 'By' 'providing' 'a'
'stable' 'and' 'small' 'core' 'system,' 'excellent' 'dev' 'tools,' 'and'
'maintained' 'releases,' 'Pharo' 'is' 'an' 'attractive' 'platform' 'to'
'build' 'and' 'deploy' 'mission' 'critical' 'Smalltalk' 'applications.')
do: [:each | s nextPutAll: each; nextPut: Character space].
s contents]] bench
--> '110,000 per second.'
[| s|
s := ''.
#('Pharo''s' 'goal' 'is' 'to' 'deliver' 'a' 'clean,' 'innovative,'
'free' 'open-source' 'Smalltalk' 'environment.' 'By' 'providing' 'a'
'stable' 'and' 'small' 'core' 'system,' 'excellent' 'dev' 'tools,' 'and'
'maintained' 'releases,' 'Pharo' 'is' 'an' 'attractive' 'platform' 'to'
'build' 'and' 'deploy' 'mission' 'critical' 'Smalltalk' 'applications.')
do: [:each | s := s , each , Character space asString].
s ] bench
--> '31,500 per second.'
]]]
Another example that may be a bit draft and fuzzy but gives an idea of
the cost that you may gain.
[[[
String streamContents: [:s |
Smalltalk globals allClasses do: [:each |
s nextPutAll: each name].
s contents]] bench
--> '246 per second.'
| s |
s := ''.
[Smalltalk globals allClasses
do: [:each | s := s, each name]] bench
--> '2.01 per second.'
]]]
!!!Preallocate when possible.
Preallocate when possible the string used with\ct{new:streamContents:}.
For example, the following method does not take advantage that the
result is always a 8 characters string.
[[[
Time>>print24
"Return as 8-digit string 'hh:mm:ss', with leading zeros if needed"
^String streamContents:
[ :aStream | self print24: true on: aStream ]
]]]
This version does the same but more efficiently, since the system does
not have to reallocate the underlying buffer.
[[[
Time>>print24
"Return as 8-digit string 'hh:mm:ss', with leading zeros if needed"
^ String new: 8 streamContents: [ :aStream |
self print24: true on: aStream ]
]]]
!!! Use ==nextPut:== when possible.
When you can, use ==nextPut:== use it instead of ==nextPutAll:==. Indeed
==nextPutAll:== requires that the argument is a container of elements.
When you have already the element, no need to create an extra container.
For example better use the second form.
[[[
stream nextPutAll: '0'
]]]
[[[
stream nextPut: $0
]]]
!!Avoid creating temporary objects
The computation of minute in Time could be written as \ct{asDuration
minutes}. However, this solution
creates a duration object that is only used to get the minutes.
In addition to be slow such approach generates extra garbage which
stresses the garbage collector.
[[[
Time>>minute
^ self asDuration minutes
Time>>asDuration
"Answer the duration since midnight"
^ Duration seconds: seconds nanoSeconds: nanos
]]]
A much better solution is to use the encapsulation of the class Time and
performs the computation locally as
[[[
Time>>minute
"Answer a number that represents the number of complete minutes in
the receiver,
after the number of complete hours has been removed."
^ (seconds rem: SecondsInHour) quo: SecondsInMinute
]]]
!! Avoid several iterations on the same collection
It may seem obvious but it is better to iterate once than two on the
same collection, when we can do what we want in a single pass.
For example, the following code creates a first string then creates
another one where the character ==$:== is removed.
[[[
Time>>hhmm24
"Return a string of the form 1123 (for 11:23 am), 2154 (for 9:54
pm), of exactly 4 digits"
^(String streamContents:
[ :aStream | self print24: true showSeconds: false on: aStream ])
copyWithout: $:
]]]
Better implement it as
[[[
Time>>hhmm24
"Return a string of the form 1123 (for 11:23 am), 2154 (for 9:54
pm), of exactly 4 digits"
^ String new: 4 streamContents: [ :aStream |
self hour printOn: aStream base: 10 length: 2 padded: true.
self minute printOn: aStream base: 10 length: 2 padded: true ]
]]]
have a look at:
http://www.slideshare.net/esug/design-principlesbehindpatagonia
Le 27/11/14 09:22, Max Leske a écrit :
On 26.11.2014, at 17:32, Ben Coman <b...@openinworld.com> wrote:
Yuriy Tymchuk wrote:
Hi everyone!
There is a Lint rule that suggests to use stream instead of strings
concatenation. What is the most common way, to create a string this way,
because the only thing I can come up with is:
String streamContents: [ :stream |
stream
nextPutAll: 'Hello';
nextPut: $ ;
nextPutAll: 'World’]
But I’m not sure if it is the preferred solution.
Cheers
Uko
This is typical advise, but previous discussion found it to be not necessarily
true.
https://www.mail-archive.com/pharo-dev@lists.pharo.org/msg08162.html
Very good advice, thanks Ben!
Perhaps the rule should be ammended/deleted.
Maybe a metric can be derived from Sven’s observations, e.g. “don’t apply if
the strings are constants” etc. I don’t think the rule should be deleted
because in the general case it’s important to understand that streams can be
more efficient.
cheers -ben
