Lisp Parentheses - coding-style

Why do Lispers format their code like shown in sample 1 instead of as shown in sample 2? To me (and I guess, to most others coming from different programming backgrounds than Lisp), the formatting shown in sample 2 would be easier to read. Is there any particular reason why Lispers prefer the sample 1 style?
Sample 1
(defun factorial (n)
(if (<= n 1)
1
(* n (factorial (- n 1)))))
Sample 2
(defun factorial (n)
(if (<= n 1)
1
(* n (factorial (- n 1)))
)
)

LISP IDE environments tend to balance parentheses automatically and manage indents based on nesting level. Sample 2 does not bring any advantages in those situations.
In the C/FORTRAN/Pascal heritage, you tend to emphasize sequencing over nesting (code parse trees are shallower and wider). End of scope is a more significant event in your code: hence emphasis has been and still to some extent is more important.

For experienced Lisp users, the nesting level is more important than finding closing parentheses. Putting closing parentheses on their own lines does not really help with nesting levels.
The basic idea is that the parentheses are directly AROUND their contents.
(a)
and not
(a
)
What follows is this:
(defun foo (bar)
(foo (bar (baz
...
)
...
)
...
)
)
vs.
(defun foo (bar)
(foo (bar (baz ...) ...) ...))
One of the basic ideas when editing Lisp text is, that you can select a list by (double-) clicking on the parentheses (or by using a key command when the cursor is inside the expression or on the parentheses). Then you can cut/copy the expression and paste it into another position in some other function. Next step is to select the other function and
re-indent the function. Done. There is no need to remove or introduce new lines for for closing parentheses. Just paste and re-indent. It just fits in. Otherwise you would either waste time formatting the code, or you would need to re-format the code with some editor tool (so that closing parentheses are on their own lines. Most of the time it that would create additional work and hinders moving code around.
There is one occasion where experienced Lispers would sometime write closing parentheses on their own line:
(defvar *persons*
(list (make-person "fred")
(make-person "jane")
(make-person "susan")
))
Here it indicates that new persons can be added. Place the cursor directly before the second closing parentheses on the last line, press c-o (open line), add the clause and indent the parentheses that they are aligned again. This saves the 'trouble' to find the right parentheses and then press return, when all parentheses are closed on one line.

Because there is no need whatsoever to line up closing parens. It doesn't add anything semantically. To know how many to close, most Lispers use a good editor, such as Emacs, that matches parens to closing parens and hence makes the task trivial.
In Python, there are no closing parens or end keywords at all, and Pythonistas live just fine.

After a while with Lisp you don't notice the parentheses any longer, so your example two comes across as having a bunch of unnecessary whitespace in the end of it. More specifically, most lispers use an editor that is aware of parentheses and takes care of closing the forms correctly, so you as the developer don't need to match opening and closing parentheses like you do in e.g. C.
As for whether the last form should be
(* n (factorial (- n 1)))
or
(* n
(factorial (- n 1)))
that mostly comes down to personal preference and how much stuff is going on in the code (In this case I'd prefer the former just because there is so little happening in the code).

Besides the fact that most Lisp IDE's use paren matching, the amount of space that you would use to write any reasonable program would be ridiculous! You would get carpal tunnel from all the scrolling. A 1,000 line program would be close to a million lines of code if you put all the closing parenthesis on their own line. It may look prettier and be easier to read in a small program, but that idea wouldn't scale well at all.

Why do C programmers write things like:
((a && b) || (c && d))
instead of
((a && b) || (c && d
)
)
Wouldn't #2 be easier to read? :)
Seriously, though, the bunched up closing style works well for other languages, too.
#include <stdlib.h>
#include <stdio.h>
int main(void)
{ int i, j, k;
for (i = 0; i < 1000; i++)
{ for (j = 0; j < 1000; j++)
{ for (k = 0; k < 1000; k++)
{ if (i * i + j * j == k * k)
{ printf("%d, %d, %d\n", i, j, k); } } } }
return 0; }
After a while, you don't "see" the braces, just the structure given by the indentation.
if/else looks like this:
if (cond)
{ stmt;
stmt; }
else
{ stmt; }
switch:
switch (expr)
{ case '3':
stmt;
break;
case '4':
{ stmt;
break; } }
structs:
struct foo
{ int x;
struct bar
{ int y; };
union u
{ int a, b;
char c; }; };

Just first and obvious reason: 4 LOC in first case vs. 7 LOC in second one.
Any text editor that is aware of LISP syntax will highlight you matched/mismatched parentheses, so it's not the problem.

I experienced the same dilemma in PHP using the CakePHP framework and its many nested array() structures, sometimes 5+ layers deep. After a short while I realized that being OCD about the closing parenthesis did nothing but waste my precious development time and distracted me from the end goal. The indentation was to be the most useful aspect of the formatting.

Saving all the space seems like the primary reason. If it's almost as readable (esp. once you're used to the syntax) why use the extra 3 lines.

Because Lisp programmers look at the indentation and "shape", not the brackets.

You might consider Lisp variants which do without the (). In Genyris it looks like this:
def factorial (n)
if (< n 2) 1
* n
factorial (- n 1)

Seems like this is not a real question, just a judgement on Lisp, but the answer is perfectly obvious (but not, apparently, to non-Lispers!): parentheses in Lisp are in no way equivalent to braces in C-like languages -- rather, they're precisely equivalent to parentheses in C-like languages. Lispers don't generally write
(foo bar baz
)
for exactly the same reason C programmers don't generally write
foo(bar, baz
);
The reason
(if foo
(bar)
(baz))
looks different from
if (foo) {
bar();
} else {
baz();
}
has more to do with the if than the parentheses!

I have an explanation as to C programmers have a problem with Lispers putting all the remaining closing parentheses at the end. The practice seems to me that the meaning of parentheses could be the reason. This explanation overlaps and expands on some other answers.
To a C programmer (or other language using {braces} like C does), Lisp looks like it uses #| comments |# instead of /* comments */. And looks like Lisp parentheses () are in place of C braces {}.
But really, the Lisp parentheses mean very close to the same thing as in C (and similar languages using parentheses). Consider
defun f () nil
This defines a function f that does absolutely nothing, nil. And I type that line exactly as is into the Lisp Listener, and it just responds "F". In other words, it accepts it, without a beginning and ending parentheses around the whole thing.
What I think happens when you type that into the Listener is the Listener passes what you typed to Eval. Passing it to Eval could be represented as:
Eval( Listener() )
And Listener accepts my input and returns what I typed. And you would consider "Listener()" replaced in the expression so that it becomes
Eval (defun f () nil)
REPL (read, eval, print, loop) could be represented conceptually as a recursion
/* REPL expressed in C */
Loop()
{
Print ( Eval ( Listen () ) );
Loop();
}
Understood in the example is that Listen(), Eval(), and Print() are defined elsewhere or are built into the language.
The Listener lets me type
setf a 5
and also lets me type
(setf a 5)
but complains when I type
((setf a 5))
If parentheses were equivalent to C language braces, then the listener would accept it. In my C\C++ compilers, I can type
void a_fun(void)
{{
}}
without complaint. I can even type
void a_fun(void)
{{{
}}}
with no complaint from the C/C++ compiler.
But if I dare to type
void a_fun((void))
{
}
my C/C++ compiler complains--just like the Lisp listener complains!
For a C programmer writing nested function calls, seems it is more natural to write
fun_d( fun_c( fun_b( fun_a())));
than to write
fun_d( fun_c( fun_b( fun_a()
)
)
);
In C, braces demarcate a block of code. And a block of code is part of a function definition. Lisp parentheses don't demarcate blocks. They are somewhat like C's parentheses. In C, the parentheses demarcate a list; maybe a list of parameters passed to a function.
In Lisp, seems the opening parentheses "(" means we are going to start a new list, to which the CAR side of a CONS construct will point. Then we list the items to be contained or optionally pointed to by the CAR side of the CONS, with the CDR side pointing to nil (for end of list) or the next CONS. The closing parenthesis ")" means to terminate the list with a nil, and go back to continue the previous level of listing. So, C language does not do CONS's. So, there is a little bit of a difference between C's parentheses and Lisp's. Even so, seems very close, maybe even practically the same thing.
Some have written that Lisp becomes more like C if you move the function outside the parentheses. For example,
(setf a 5)
becomes
setf(a 5)
But what it really is, is that Eval requires that the first item in a list be the function that Eval needs to call. It is more like passing a C function a pointer to a function. So, what it really is is
eval(setf a 5)
And that looks more like C. You can even type it in the listener just like that without complaint from the listener or eval. You are just saying that eval should call eval which should then call setf. The rest of the list are parameters to setf.
Anyway, that's just what I think I see.
It's just that Eval needs to know which processor to call.
And I think this explanation provides an understanding of why C programmers think initially that Lispers should align closing parentheses under the opening parentheses which they close. The C programmer mistakenly thinks that Lisp's parenthesis correspond to C's braces. They don't. Lisp's parentheses correspond to C's parentheses.

Related

call function with two string parameters in Common-LISP Programming

I created a function which gets two string parameters. The function simply adds each string length. below is a code.
(defun add_twostring_length (mystr1 mystr2)
(+ (length mystr1) (length mystr2))
)
When I call add_twostring_length function like this,
(add_twostring_length "cpp" "lisp")
output is correct. 7
But, when I call the same function in the manner of using comma,
(add_twostring_length "cpp", "lisp")
I got an error message.
Error: Comma not inside a backquote.
[condition type: READER-ERROR]
I want to call function in the manner of (add_twostring_length "cpp", "lisp").
What is the wrong with the code?
picture showing error message
You might as well ask "why can't I call the function without parentheses?" In lisp, you call functions as an sexpr with the function in the car and the arguments in the cdr. There are no commas involved -- that's the syntax of lisp.
What you want is possible, but I will strongly advice against using it:
(set-macro-character #\,
#'(lambda (stream char)
(read stream t nil t)))
The above code creates the so called "read macro". At read-time common lisp will find all occurrences of , and ignore them. This makes possible calling functions like so:
(+ 1, 2, 3) ; => 6
However this will break escaping in templates:
`(1 2 ,(+ 3 4)) ; => (1 2 (+ 3 4))
Perhaps it is possible to make the read macro more intelligent, but I don't want to delve deeper into this, because I don't like the idea. Sorry.

Haskell debugging an arbitrary lambda expression

I have a set of lambda expressions which I'm passing to other lambdas. All lambdas rely only on their arguments, they don't call any outside functions. Of course, sometimes it gets quite confusing and I'll pass an function with the incorrect number of arguments to another, creating a GHCi exception.
I want to make a debug function which will take an arbitrary lambda expression (with an unknown number of arguments) and return a string based on the structure and function of the lambda.
For example, say I have the following lambda expressions:
i = \x -> x
k = \x y -> x
s = \x y z -> x z (y z)
debug (s k) should return "\a b -> b"
debug (s s k) should return "\a b -> a b a" (if I simplified that correctly)
debug s should return "\a b c -> a c (b c)"
What would be a good way of doing this?
I think the way to do this would be to define a small lambda calculus DSL in Haskell (or use an existing implementation). This way, instead of using the native Haskell formulation, you would write something like
k = Lam "x" (Lam "y" (App (Var "x") (Var "y")))
s = Lam "x" (Lam "y" (Lam "z" (App (App (Var "x") (Var "z")
(App (Var "y") (Var "z"))))
and similarly for s and i. You would then write/use an evaluation function so that you could write
debug e = eval e
debug (App s k)
which would give you the final form in your own syntax. Additionally you would need a sort of interpreter to convert your DSL syntax to Haskell, so that you can actually use the functions in your code.
Implementing this does seem like quite a lot of (tricky) work, and it's probably not exactly what you had in mind (especially if you need the evaluation for typed syntax), but I'm sure it would be a great learning experience. A good reference would be chapter 6 of "Write you a Haskell". Using an existing implementation would be a lot easier (but less fun :)).
If this is merely for debugging purposes you might benefit from looking at the core syntax ghc compiles to. See chapter 25 of Real world Haskell, the ghc flag to use is -ddump-simpl. But this would mean looking at generated code rather than generating a representation inside your program. I'm also not sure to what extent you would be able to identify specific functions in the Core code easily (I have no experience with this so YMMV).
It would of course be pretty cool if using show on functions would give the kind of output you describe but there are probably very good reasons functions are not an instance of Show (I wouldn't be able to tell you).
You can actually achieve that by utilising pretty-printing from Template Haskell, which comes with GHC out of the box.
First, the formatting function should be defined in separate module (that's a TH restriction):
module LambdaPrint where
import Control.Monad
import Language.Haskell.TH.Ppr
import Language.Haskell.TH.Syntax
showDef :: Name -> Q Exp
showDef = liftM (LitE . StringL . pprint) . reify
Then use it:
{-# LANGUAGE TemplateHaskell #-}
import LambdaPrint
y :: a -> a
y = \a -> a
$(return []) --workaround for GHC 7.8+
test = $(showDef 'y)
The result is more or less readable, not counting fully qualified names:
*Main> test
"Main.y :: forall a_0 . a_0 -> a_0"
Few words about what's going on. showDef is a macro function which reifies the definition of some name from the environment and pretty-prints it in a string literal expression. To use it, you need to quote the name of the lambda (using ') and splice the result (which is a quoted string expression) into some expression (using $(...)).

UU-Parsinglib slowering drastically when some rules are enabled

I'm writing a compiler using uu-parsinglib and I saw a very strange thing. I defined a pChoice combinator like:
pChoice = foldr (<<|>) pFail
(notice, I'm using greedy <<|>).
Lets consider following code:
pFactor i = pChoice [ Expr.Var <$> pVar
, Expr.Lit <$> pLit True
, L.pParensed (pExpr i)
-- , Expr.Tuple <$> pTuple (pOpE i)
-- , Expr.List <$> pLst (pListE i)
]
Each element starts with different character - Expr.Var starts with a letter, Expr.Lit with a number, L.pParensed with parenthesis (, Expr.Tuple with brace { and Expr.List with bracket [.
I've got a big test code in which there are no tuples and no lists. The code parses in 0.15s. When I uncomment the above lines, the time increases to 0.65s. This is over 400% slowdown... How is it possible? I'm using only greedy operators and I'm sure parser is not haning in Tuple nor List section, because in the whole code there is no tuple nor list.
If you would need more code or definitions, I'll of course poste it.
I think the cause of the matter may lie in the fact that you have parameterised pFactor. This will cause each call to such a parser to build a new parser, which will take time. It is much better to create such parsers once and for all and share them in the actual parsing process. I cannot see how you are using this parser I cannot answer your questions any further.

Evaluation in Scheme

ok, i have this problem here. i was asked to write a function in Scheme, that takes an "environment", and an expression and it returns the value of this expression, for the variable bindings found in the enviroment.
and a definition of a boolean expression is this according to the question below.
edit sorry my question is what does it mean by "it takes an environment" as an argument and what exactly does the function need to do?
evaluate for example "T OR F" and return "F" ???
"<expr> ::= <boolean>
|<variable>
|(not <expr>)
|(or <expr> <expr>)
|(and <expr> <expr>"
An environment is basically a dictionary of variable names to values. So given the environment
var1 = #t
var2 = #f
var3 = #t
and an expression
(or var2 (and T (or var1 var3)))
You would need to substitute the given values of var1, var2, and var3 into the expression, and then evaluate the expression.
Your function will probably be given the environment as some sort of Lisp structure, probably an alist, as one of its parameters.
From what I can determine you have been asked to implement eval.
So you should for starters have:
(define (eval expr env)
... )
where expr can be any of the forms you mention and env would keep the symbols defined (possibly a association list).
The first 2 cases are relatively trivial, the 3rd one, application of the not procedure should also be easy, given not will be in the environment (eg (list (cons 'not not))).
But the harder part lies in the last 2. Both of those are macros, and will require some expansion. The standard definitions/expansions of those should have been given to you. Once expanded, you can simply call eval recursively to evaluate the expanded expression.
Good luck :)
Edit:
Both and and or expands to if, so you will need to implement that too.
By "environment" they probably mean the equivalent of "scope" in other languages. Consider the following C fragment:
if (7 < 100)
{
int j = 2;
if (j < 4)
{
int k = 7, j = 14;
printf("k = %d, j = %d\n", k, j);
}
}
Note that in the outer scope (marked out by the outer set of braces) the only variable is j. In the inner scope there is a new j and a k. So there are three variables here, the outer j, and the inner j and k.
One way of implementing this is to define a scope to be a list of "environments". As you enter a new block, you put another "environment" in your list. When looking up variables by name, you look first in the most recently added "environment". If it isn't found there, you move along the list of environments to the next and look there, and so on.
An "environment" itself is often just a list of pairs, matching up names of variables with values. So it sounds like you are being asked to pass such a list to your function, each pair giving the symbol for a boolean variable and its value. Based on which variables are currently "in scope", you fetch their values out of the environment and use them in the expressions you are evaluating (according to that expression grammar you've been given).
In your case, it sounds like you aren't being asked to worry about which enviroments are in scope. You just have one environment, i.e. one list of pairs.
Sounds like a fair bit of work, good luck!
One reference that might help is:
http://michaux.ca/articles/scheme-from-scratch-bootstrap-v0_9-environments

What are the precise rules for when you can omit parenthesis, dots, braces, = (functions), etc.?

What are the precise rules for when you can omit (omit) parentheses, dots, braces, = (functions), etc.?
For example,
(service.findAllPresentations.get.first.votes.size) must be equalTo(2).
service is my object
def findAllPresentations: Option[List[Presentation]]
votes returns List[Vote]
must and be are both functions of specs
Why can't I go:
(service findAllPresentations get first votes size) must be equalTo(2)
?
The compiler error is:
"RestServicesSpecTest.this.service.findAllPresentations
of type
Option[List[com.sharca.Presentation]]
does not take parameters"
Why does it think I'm trying to pass in a parameter? Why must I use dots for every method call?
Why must (service.findAllPresentations get first votes size) be equalTo(2) result in:
"not found: value first"
Yet, the "must be equalTo 2" of
(service.findAllPresentations.get.first.votes.size) must be equalTo 2, that is, method chaining works fine? - object chain chain chain param.
I've looked through the Scala book and website and can't really find a comprehensive explanation.
Is it in fact, as Rob H explains in Stack Overflow question Which characters can I omit in Scala?, that the only valid use-case for omitting the '.' is for "operand operator operand" style operations, and not for method chaining?
You seem to have stumbled upon the answer. Anyway, I'll try to make it clear.
You can omit dot when using the prefix, infix and postfix notations -- the so called operator notation. While using the operator notation, and only then, you can omit the parenthesis if there is less than two parameters passed to the method.
Now, the operator notation is a notation for method-call, which means it can't be used in the absence of the object which is being called.
I'll briefly detail the notations.
Prefix:
Only ~, !, + and - can be used in prefix notation. This is the notation you are using when you write !flag or val liability = -debt.
Infix:
That's the notation where the method appears between an object and it's parameters. The arithmetic operators all fit here.
Postfix (also suffix):
That notation is used when the method follows an object and receives no parameters. For example, you can write list tail, and that's postfix notation.
You can chain infix notation calls without problem, as long as no method is curried. For example, I like to use the following style:
(list
filter (...)
map (...)
mkString ", "
)
That's the same thing as:
list filter (...) map (...) mkString ", "
Now, why am I using parenthesis here, if filter and map take a single parameter? It's because I'm passing anonymous functions to them. I can't mix anonymous functions definitions with infix style because I need a boundary for the end of my anonymous function. Also, the parameter definition of the anonymous function might be interpreted as the last parameter to the infix method.
You can use infix with multiple parameters:
string substring (start, end) map (_ toInt) mkString ("<", ", ", ">")
Curried functions are hard to use with infix notation. The folding functions are a clear example of that:
(0 /: list) ((cnt, string) => cnt + string.size)
(list foldLeft 0) ((cnt, string) => cnt + string.size)
You need to use parenthesis outside the infix call. I'm not sure the exact rules at play here.
Now, let's talk about postfix. Postfix can be hard to use, because it can never be used anywhere except the end of an expression. For example, you can't do the following:
list tail map (...)
Because tail does not appear at the end of the expression. You can't do this either:
list tail length
You could use infix notation by using parenthesis to mark end of expressions:
(list tail) map (...)
(list tail) length
Note that postfix notation is discouraged because it may be unsafe.
I hope this has cleared all the doubts. If not, just drop a comment and I'll see what I can do to improve it.
Class definitions:
val or var can be omitted from class parameters which will make the parameter private.
Adding var or val will cause it to be public (that is, method accessors and mutators are generated).
{} can be omitted if the class has no body, that is,
class EmptyClass
Class instantiation:
Generic parameters can be omitted if they can be inferred by the compiler. However note, if your types don't match, then the type parameter is always infered so that it matches. So without specifying the type, you may not get what you expect - that is, given
class D[T](val x:T, val y:T);
This will give you a type error (Int found, expected String)
var zz = new D[String]("Hi1", 1) // type error
Whereas this works fine:
var z = new D("Hi1", 1)
== D{def x: Any; def y: Any}
Because the type parameter, T, is inferred as the least common supertype of the two - Any.
Function definitions:
= can be dropped if the function returns Unit (nothing).
{} for the function body can be dropped if the function is a single statement, but only if the statement returns a value (you need the = sign), that is,
def returnAString = "Hi!"
but this doesn't work:
def returnAString "Hi!" // Compile error - '=' expected but string literal found."
The return type of the function can be omitted if it can be inferred (a recursive method must have its return type specified).
() can be dropped if the function doesn't take any arguments, that is,
def endOfString {
return "myDog".substring(2,1)
}
which by convention is reserved for methods which have no side effects - more on that later.
() isn't actually dropped per se when defining a pass by name paramenter, but it is actually a quite semantically different notation, that is,
def myOp(passByNameString: => String)
Says myOp takes a pass-by-name parameter, which results in a String (that is, it can be a code block which returns a string) as opposed to function parameters,
def myOp(functionParam: () => String)
which says myOp takes a function which has zero parameters and returns a String.
(Mind you, pass-by-name parameters get compiled into functions; it just makes the syntax nicer.)
() can be dropped in the function parameter definition if the function only takes one argument, for example:
def myOp2(passByNameString:(Int) => String) { .. } // - You can drop the ()
def myOp2(passByNameString:Int => String) { .. }
But if it takes more than one argument, you must include the ():
def myOp2(passByNameString:(Int, String) => String) { .. }
Statements:
. can be dropped to use operator notation, which can only be used for infix operators (operators of methods that take arguments). See Daniel's answer for more information.
. can also be dropped for postfix functions
list tail
() can be dropped for postfix operators
list.tail
() cannot be used with methods defined as:
def aMethod = "hi!" // Missing () on method definition
aMethod // Works
aMethod() // Compile error when calling method
Because this notation is reserved by convention for methods that have no side effects, like List#tail (that is, the invocation of a function with no side effects means that the function has no observable effect, except for its return value).
() can be dropped for operator notation when passing in a single argument
() may be required to use postfix operators which aren't at the end of a statement
() may be required to designate nested statements, ends of anonymous functions or for operators which take more than one parameter
When calling a function which takes a function, you cannot omit the () from the inner function definition, for example:
def myOp3(paramFunc0:() => String) {
println(paramFunc0)
}
myOp3(() => "myop3") // Works
myOp3(=> "myop3") // Doesn't work
When calling a function that takes a by-name parameter, you cannot specify the argument as a parameter-less anonymous function. For example, given:
def myOp2(passByNameString:Int => String) {
println(passByNameString)
}
You must call it as:
myOp("myop3")
or
myOp({
val source = sourceProvider.source
val p = myObject.findNameFromSource(source)
p
})
but not:
myOp(() => "myop3") // Doesn't work
IMO, overuse of dropping return types can be harmful for code to be re-used. Just look at specification for a good example of reduced readability due to lack of explicit information in the code. The number of levels of indirection to actually figure out what the type of a variable is can be nuts. Hopefully better tools can avert this problem and keep our code concise.
(OK, in the quest to compile a more complete, concise answer (if I've missed anything, or gotten something wrong/inaccurate please comment), I have added to the beginning of the answer. Please note this isn't a language specification, so I'm not trying to make it exactly academically correct - just more like a reference card.)
A collection of quotes giving insight into the various conditions...
Personally, I thought there'd be more in the specification. I'm sure there must be, I'm just not searching for the right words...
There are a couple of sources however, and I've collected them together, but nothing really complete / comprehensive / understandable / that explains the above problems to me...:
"If a method body has more than one
expression, you must surround it with
curly braces {…}. You can omit the
braces if the method body has just one
expression."
From chapter 2, "Type Less, Do More", of Programming Scala:
"The body of the upper method comes
after the equals sign ‘=’. Why an
equals sign? Why not just curly braces
{…}, like in Java? Because semicolons,
function return types, method
arguments lists, and even the curly
braces are sometimes omitted, using an
equals sign prevents several possible
parsing ambiguities. Using an equals
sign also reminds us that even
functions are values in Scala, which
is consistent with Scala’s support of
functional programming, described in
more detail in Chapter 8, Functional
Programming in Scala."
From chapter 1, "Zero to Sixty: Introducing Scala", of Programming Scala:
"A function with no parameters can be
declared without parentheses, in which
case it must be called with no
parentheses. This provides support for
the Uniform Access Principle, such
that the caller does not know if the
symbol is a variable or a function
with no parameters.
The function body is preceded by "="
if it returns a value (i.e. the return
type is something other than Unit),
but the return type and the "=" can be
omitted when the type is Unit (i.e. it
looks like a procedure as opposed to a
function).
Braces around the body are not
required (if the body is a single
expression); more precisely, the body
of a function is just an expression,
and any expression with multiple parts
must be enclosed in braces (an
expression with one part may
optionally be enclosed in braces)."
"Functions with zero or one argument
can be called without the dot and
parentheses. But any expression can
have parentheses around it, so you can
omit the dot and still use
parentheses.
And since you can use braces anywhere
you can use parentheses, you can omit
the dot and put in braces, which can
contain multiple statements.
Functions with no arguments can be
called without the parentheses. For
example, the length() function on
String can be invoked as "abc".length
rather than "abc".length(). If the
function is a Scala function defined
without parentheses, then the function
must be called without parentheses.
By convention, functions with no
arguments that have side effects, such
as println, are called with
parentheses; those without side
effects are called without
parentheses."
From blog post Scala Syntax Primer:
"A procedure definition is a function
definition where the result type and
the equals sign are omitted; its
defining expression must be a block.
E.g., def f (ps) {stats} is
equivalent to def f (ps): Unit =
{stats}.
Example 4.6.3 Here is a declaration
and a de?nition of a procedure named
write:
trait Writer {
def write(str: String)
}
object Terminal extends Writer {
def write(str: String) { System.out.println(str) }
}
The code above is implicitly completed
to the following code:
trait Writer {
def write(str: String): Unit
}
object Terminal extends Writer {
def write(str: String): Unit = { System.out.println(str) }
}"
From the language specification:
"With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses, enabling the operator
syntax shown in our insertion operator
example. This syntax is used in other
places in the Scala API, such as
constructing Range instances:
val firstTen:Range = 0 to 9
Here again, to(Int) is a vanilla
method declared inside a class
(there’s actually some more implicit
type conversions here, but you get the
drift)."
From Scala for Java Refugees Part 6: Getting Over Java:
"Now, when you try "m 0", Scala
discards it being a unary operator, on
the grounds of not being a valid one
(~, !, - and +). It finds that "m" is
a valid object -- it is a function,
not a method, and all functions are
objects.
As "0" is not a valid Scala
identifier, it cannot be neither an
infix nor a postfix operator.
Therefore, Scala complains that it
expected ";" -- which would separate
two (almost) valid expressions: "m"
and "0". If you inserted it, then it
would complain that m requires either
an argument, or, failing that, a "_"
to turn it into a partially applied
function."
"I believe the operator syntax style
works only when you've got an explicit
object on the left-hand side. The
syntax is intended to let you express
"operand operator operand" style
operations in a natural way."
Which characters can I omit in Scala?
But what also confuses me is this quote:
"There needs to be an object to
receive a method call. For instance,
you cannot do “println “Hello World!”"
as the println needs an object
recipient. You can do “Console
println “Hello World!”" which
satisfies the need."
Because as far as I can see, there is an object to receive the call...
I find it easier to follow this rule of thumb: in expressions spaces alternate between methods and parameters. In your example, (service.findAllPresentations.get.first.votes.size) must be equalTo(2) parses as (service.findAllPresentations.get.first.votes.size).must(be)(equalTo(2)). Note that the parentheses around the 2 have a higher associativity than the spaces. Dots also have higher associativity, so (service.findAllPresentations.get.first.votes.size) must be.equalTo(2)would parse as (service.findAllPresentations.get.first.votes.size).must(be.equalTo(2)).
service findAllPresentations get first votes size must be equalTo 2 parses as service.findAllPresentations(get).first(votes).size(must).be(equalTo).2.
Actually, on second reading, maybe this is the key:
With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses
As mentioned on the blog post: http://www.codecommit.com/blog/scala/scala-for-java-refugees-part-6 .
So perhaps this is actually a very strict "syntax sugar" which only works where you are effectively calling a method, on an object, which takes one parameter. e.g.
1 + 2
1.+(2)
And nothing else.
This would explain my examples in the question.
But as I said, if someone could point out to be exactly where in the language spec this is specified, would be great appreciated.
Ok, some nice fellow (paulp_ from #scala) has pointed out where in the language spec this information is:
6.12.3:
Precedence and associativity of
operators determine the grouping of
parts of an expression as follows.
If there are several infix operations in an expression, then
operators with higher precedence bind
more closely than operators with lower
precedence.
If there are consecutive infix operations e0 op1 e1 op2 . . .opn en
with operators op1, . . . , opn of the
same precedence, then all these
operators must have the same
associativity. If all operators are
left-associative, the sequence is
interpreted as (. . . (e0 op1 e1) op2
. . .) opn en. Otherwise, if all
operators are rightassociative, the
sequence is interpreted as e0 op1 (e1
op2 (. . .opn en) . . .).
Postfix operators always have lower precedence than infix operators. E.g.
e1 op1 e2 op2 is always equivalent to
(e1 op1 e2) op2.
The right-hand operand of a
left-associative operator may consist
of several arguments enclosed in
parentheses, e.g. e op (e1, . . .
,en). This expression is then
interpreted as e.op(e1, . . . ,en).
A left-associative binary operation e1
op e2 is interpreted as e1.op(e2). If
op is rightassociative, the same
operation is interpreted as { val
x=e1; e2.op(x ) }, where x is a fresh
name.
Hmm - to me it doesn't mesh with what I'm seeing or I just don't understand it ;)
There aren't any. You will likely receive advice around whether or not the function has side-effects. This is bogus. The correction is to not use side-effects to the reasonable extent permitted by Scala. To the extent that it cannot, then all bets are off. All bets. Using parentheses is an element of the set "all" and is superfluous. It does not provide any value once all bets are off.
This advice is essentially an attempt at an effect system that fails (not to be confused with: is less useful than other effect systems).
Try not to side-effect. After that, accept that all bets are off. Hiding behind a de facto syntactic notation for an effect system can and does, only cause harm.

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