[squeak-dev] The Trunk: Compiler-eem.480.mcz

Eliot Miranda eliot.miranda at gmail.com
Wed Nov 23 15:36:09 UTC 2022

Hi Christoph, Hi Marcel,

apologies about the font size mismatches...

On Wed, Nov 23, 2022 at 2:25 AM Marcel Taeumel <marcel.taeumel at hpi.de>

> Hi Christoph --
> >  IMHO, it unnecessarily complicates the simple Smalltalk syntax.  [...]
> Nah, this is just a tooling change, not a syntactical one.


> Yes, I would like to have this info skipped for #isNil as well. Note that one
> should not use  #ifNotNilDo: anymore.

Good idea.  I'll include it.

> Best,
> Marcel
> Am 23.11.2022 11:00:43 schrieb Thiede, Christoph <
> christoph.thiede at student.hpi.uni-potsdam.de>:
> Hi Eliot, hi all,
> I'm skeptical about this change, as it creates or expands a special role
> of the selectors #ifNil:, #ifNotNil:, and their combinations. IMHO, it
> unnecessarily complicates the simple Smalltalk syntax. While I know and
> sometimes dislike these UndefinedVariable notifications, too, I don't know
> whether differentiating them by the selector is the right strategy
> to improve this situation.
> Please indulge me.  It's f***ing irritating to be told by the compiler
that as temp var appears to be uninitialized when one is intentionally
using the fact that temps are initialized to nil.  And that temp vars are
initialized to nil is a) essential knowledge and b) a good thing (no
uninitialized local variables a la C, a sensible value to initialize a
variable with).

BTW, I find it more than sad (a little alarming in fact) that
someSmalltalkers don't know that the value of several conditionals that
take blocks is nil when the condition doesn't select the block. e.g. false
ifTrue: [self anything] is nil.  I see "expr ifNotNil: [...] ifNil: [nil]"
and it strikes me as illiterate.  I recently visited code written by a
strong programmer who open coded a lot of point arithmetic, decomposing
e.g. a * b into (a x * b x) @ (a y * b y). It's bad.  It gradually
degrades the code base in that it isn't always an exemplar of best

> Consider the following examples:
> | a b c d e f g h |
> a ifNil: [a := 1].
> c := b.
> c ifNil: [c := 3].
> #(1 2 3) sorted: d.
> e := 5.
> (e isNil or: [f isNil]) ifTrue: [e := f := 6].
> g perform: #ifNotNil: with: [b := g].
> h ifNotNilDo: [h := 8].
> How would you explain to a naive Smalltalker which of these variables will
> be marked as undefined at this point and why? (Of course, you can explain
> it by pointing to the implementation, but I think that's a significantly
> less intuitive explanation than just saying "you must declare any variable
> before using it".)
No. It's a hard-and-fast rule that all temp vars are initialized to nil.
And initializing a variable (to other than nil) is done by assigning it.
In the above a through h are declared within the vertical bars.n They are
initialized in the assignments.  I want a warning for the usage of b in "c
:= b", "d" in "#(1 2 3) sorted: d", g in "g perform: #ifNotNil: with: [b :=
g]".  I *don't* want to be told about a in "a ifNil: [a := 1]", c in "c
ifNil: [c := 3]", or e & f in "(e isNil or: [f isNil]) ifTrue: [e := f :=
6]".  I never want to see "ifNotNilDo", ever ;-)
(* note that a couple of years back we fixed a bad bug in the compiler
where block local temps were  not (re)initialized to nil on each iteration,
leaking their values from previous iterations, breaking the "all temp vars
are initialized to nil rule, and revealing implementation details in the
compiler's inlining of to:[by:]do: forms)

> This behavior leads to a mental model that disambiguates between null and
> undefined similar to JavaScript which I never have found helpful.
I don't see how that applies.  Smalltalk has no undefined.  It has nil &
zero, and these values are used to initialize any and all variables.  This
is not an artifact of the implementation.  It is a fundamental part of the
language design.  It results in no dangling referents or
uninitialized variables.  The language used in Parser>>#queryUndefined is
problematic. It should be "unassigned", not "undefined". There is nothing
undefined about these variables.  But they are indeed unassigned.  In some
cases (see my i=diomatic implementation of subsequences: and substrings)
this can (and *should*) be used to advantage.  And all Smalltalk
programming courses should explain that variables are always initialized
(either to nil or zero, & hence by extension 0.0, Character null, Color
transparent, et al), and may need assignment before their referents get
sent messages.

I see the same kind of sloppiness in people not knowing that conditionals
that take blocks typically evaluate to nil when the condition doesn;t
select the block.  So always "expr ifNotNil: [...]", never "expr ifNotNil:
[...] ifNil: [nil]", or "expr ifNotNil: [...] ifNil: []". I recently
cleaned up code by as string programmer who had open coded point arithmetic
(e.g. a * b written as (a x * b x) @ (a y * b y) ).  This is really bad:
it's exemplifying poor practice, it's verbose, it takes away at least as
much understanding as it conveys, it leads to more difficult to manage code.

If we fail to teach the language properly we start on a slippery slope to
duplication (which is an awful evil, leading to much increased maintennance
effort, and brittleness), and rendering perfectly good, well thought-out
idioms mysterious.  It;'s not like Smalltalk has a lot of rules; the
number, compared to C & C++ et al is tiny.  And terseness has not just
aesthetic benefit, but real practical benefit in terms of readability &

> Also, with this change, the compiler leaks the default value of any
> temporary variable, which we previously were able to hide at least
> partially.
But that is a MISTAKE!! The language designers didn't arrange for temps to
be initialized to nil just because that's the only default.  They did it to
ensure that there is no such thing as an  uninitialized variable in
Smalltalk.  That's why nil ids an object, with a class, not just nil.
That's why nil ~~ false.  It's carefully thought out and not just some
artifact of the implementation.  And that rationale (read the blue book
carefully) and its implications, should be taught/learned/known, and
especially exemplified by the core code of Squeak trunk, and hence
supported by the compiler.

> In many cases, I think explicitly setting a temporary variable to nil
> before it is initialized within some non-trivial conditional complex would
> be more explicit, thus more readable, and something which we should
> generally encourage programmers to do.
I disagree.  You're advocating for absurdities such as

    | colors |
    colors :=- ColorArray new: 256.
    colors atAllPut: Color transparent

This is the kind of thinking that leads to cycling wearing American
Football clothes.  It won't keep you from being run over by a truck, but
it'll make you so slow and reduce your peripheral vision so much, not to
mention give you a false sense of security, that you'll be much more likely
to be run over by a truck...

> Looking forward to your opinion!
:-)  Hope I'm not too strident :-)

> Best,
> Christoph
> ------------------------------
> *Von:* Squeak-dev <squeak-dev-bounces at lists.squeakfoundation.org> im
> Auftrag von commits at source.squeak.org <commits at source.squeak.org>
> *Gesendet:* Mittwoch, 23. November 2022 04:10:30
> *An:* squeak-dev at lists.squeakfoundation.org;
> packages at lists.squeakfoundation.org
> *Betreff:* [squeak-dev] The Trunk: Compiler-eem.480.mcz
> Eliot Miranda uploaded a new version of Compiler to project The Trunk:
> http://source.squeak.org/trunk/Compiler-eem.480.mcz
> ==================== Summary ====================
> Name: Compiler-eem.480
> Author: eem
> Time: 22 November 2022, 7:10:27.324796 pm
> UUID: 3e5ba19e-c44a-4390-9004-de1246736cbc
> Ancestors: Compiler-eem.479
> Do not warn of an uninitialized temporary if it is being sent ifNil: or
> ifNotNil:.
> =============== Diff against Compiler-eem.479 ===============
> Item was changed:
>   ----- Method: Parser>>primaryExpression (in category 'expression types')
> -----
>   primaryExpression
>          hereType == #word
>                  ifTrue:
>                          [parseNode := self variable.
> +                        (parseNode isUndefTemp
> +                         and: [(#('ifNil:' 'ifNotNil:') includes: here)
> not
> +                         and: [self interactive]])
> +                                ifTrue:
> +                                        [self queryUndefined].
> -                        (parseNode isUndefTemp and: [self interactive])
> -                                ifTrue: [self queryUndefined].
>                          parseNode nowHasRef.
>                          ^ true].
>          hereType == #leftBracket
>                  ifTrue:
>                          [self advance.
>                          self blockExpression.
>                          ^true].
>          hereType == #leftBrace
>                  ifTrue:
>                          [self braceExpression.
>                          ^true].
>          hereType == #leftParenthesis
>                  ifTrue:
>                          [self advance.
>                          self expression ifFalse: [^self expected:
> 'expression'].
>                          (self match: #rightParenthesis)
>                                  ifFalse: [^self expected: 'right
> parenthesis'].
>                          ^true].
>          (hereType == #string or: [hereType == #number or: [hereType ==
> #literal or: [hereType == #character]]])
>                  ifTrue:
>                          [parseNode := encoder encodeLiteral: self advance.
>                          ^true].
>          (here == #- and: [tokenType == #number and: [1 + hereEnd = mark]])
>                  ifTrue:
>                          [self advance.
>                          parseNode := encoder encodeLiteral: self advance
> negated.
>                          ^true].
>          ^false!

best, Eliot
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