I'm curious as to when evaluation sets in, apparently certain operators are rather transformed into clauses than evaluated:
abstract sig Element {}
one sig A,B,C extend Element {}
one sig Test {
test: set Element
}
pred Test1 { Test.test = A+B }
pred Test2 { Test.test = Element-C }
and run it for Test1 and Test2 respectively will give different number of vars/clauses, specifically:
Test1: 0 vars, 0 primary vars, 0 clauses
Test2: 5 vars, 3 primary vars, 4 clauses
So although Element is abstract and all its members and their cardinalities are known, the difference seems not to be computed in advance, while the sum is. I don't want to make any assumptions, so I'm interested in why that is. Is the + operator special?
To give some context, I tried to limit the domain of a relation and found, that using only + seems to be more efficient, even when the sets are completely known in advance.
To give some context, I tried to limit the domain of a relation and found, that using only + seems to be more efficient, even when the sets are completely known in advance.
That is pretty much the right conclusion. The reason is the fact that the Alloy Analyzer tries to infer relation bounds from certain Alloy idioms. It uses a conservative approximation that is always sound for set union and product, but not for set difference. That's why for Test1 in the example above the Alloy Analyzer infers a fixed bound for the test relation (this/Test.test: [[[A$0], [B$0]]]) so no solver needs to be invoked; for Test2, the bound for the test relation cannot be shrunk so is set to be the most permissive (this/Test.test: [[], [[A$0], [B$0], [C$0]]]), thus a solver needs to be invoked to find a solution satisfying the constraints given the bounds.
Is there a typed programming language where I can constrain types like the following two examples?
A Probability is a floating point number with minimum value 0.0 and maximum value 1.0.
type Probability subtype of float
where
max_value = 0.0
min_value = 1.0
A Discrete Probability Distribution is a map, where: the keys should all be the same type, the values are all Probabilities, and the sum of the values = 1.0.
type DPD<K> subtype of map<K, Probability>
where
sum(values) = 1.0
As far as I understand, this is not possible with Haskell or Agda.
What you want is called refinement types.
It's possible to define Probability in Agda: Prob.agda
The probability mass function type, with sum condition is defined at line 264.
There are languages with more direct refinement types than in Agda, for example ATS
You can do this in Haskell with Liquid Haskell which extends Haskell with refinement types. The predicates are managed by an SMT solver at compile time which means that the proofs are fully automatic but the logic you can use is limited by what the SMT solver handles. (Happily, modern SMT solvers are reasonably versatile!)
One problem is that I don't think Liquid Haskell currently supports floats. If it doesn't though, it should be possible to rectify because there are theories of floating point numbers for SMT solvers. You could also pretend floating point numbers were actually rational (or even use Rational in Haskell!). With this in mind, your first type could look like this:
{p : Float | p >= 0 && p <= 1}
Your second type would be a bit harder to encode, especially because maps are an abstract type that's hard to reason about. If you used a list of pairs instead of a map, you could write a "measure" like this:
measure total :: [(a, Float)] -> Float
total [] = 0
total ((_, p):ps) = p + probDist ps
(You might want to wrap [] in a newtype too.)
Now you can use total in a refinement to constrain a list:
{dist: [(a, Float)] | total dist == 1}
The neat trick with Liquid Haskell is that all the reasoning is automated for you at compile time, in return for using a somewhat constrained logic. (Measures like total are also very constrained in how they can be written—it's a small subset of Haskell with rules like "exactly one case per constructor".) This means that refinement types in this style are less powerful but much easier to use than full-on dependent types, making them more practical.
Perl6 has a notion of "type subsets" which can add arbitrary conditions to create a "sub type."
For your question specifically:
subset Probability of Real where 0 .. 1;
and
role DPD[::T] {
has Map[T, Probability] $.map
where [+](.values) == 1; # calls `.values` on Map
}
(note: in current implementations, the "where" part is checked at run-time, but since "real types" are checked at compile-time (that includes your classes), and since there are pure annotations (is pure) inside the std (which is mostly perl6) (those are also on operators like *, etc), it's only a matter of effort put into it (and it shouldn't be much more).
More generally:
# (%% is the "divisible by", which we can negate, becoming "!%%")
subset Even of Int where * %% 2; # * creates a closure around its expression
subset Odd of Int where -> $n { $n !%% 2 } # using a real "closure" ("pointy block")
Then you can check if a number matches with the Smart Matching operator ~~:
say 4 ~~ Even; # True
say 4 ~~ Odd; # False
say 5 ~~ Odd; # True
And, thanks to multi subs (or multi whatever, really – multi methods or others), we can dispatch based on that:
multi say-parity(Odd $n) { say "Number $n is odd" }
multi say-parity(Even) { say "This number is even" } # we don't name the argument, we just put its type
#Also, the last semicolon in a block is optional
Nimrod is a new language that supports this concept. They are called Subranges. Here is an example. You can learn more about the language here link
type
TSubrange = range[0..5]
For the first part, yes, that would be Pascal, which has integer subranges.
The Whiley language supports something very much like what you are saying. For example:
type natural is (int x) where x >= 0
type probability is (real x) where 0.0 <= x && x <= 1.0
These types can also be implemented as pre-/post-conditions like so:
function abs(int x) => (int r)
ensures r >= 0:
//
if x >= 0:
return x
else:
return -x
The language is very expressive. These invariants and pre-/post-conditions are verified statically using an SMT solver. This handles examples like the above very well, but currently struggles with more complex examples involving arrays and loop invariants.
For anyone interested, I thought I'd add an example of how you might solve this in Nim as of 2019.
The first part of the questions is straightfoward, since in the interval since since this question was asked, Nim has gained the ability to generate subrange types on floats (as well as ordinal and enum types). The code below defines two new float subranges types, Probability and ProbOne.
The second part of the question is more tricky -- defining a type with constrains on a function of it's fields. My proposed solution doesn't directly define such a type but instead uses a macro (makePmf) to tie the creation of a constant Table[T,Probability] object to the ability to create a valid ProbOne object (thus ensuring that the PMF is valid). The makePmf macro is evaluated at compile time, ensuring that you can't create an invalid PMF table.
Note that I'm a relative newcomer to Nim so this may not be the most idiomatic way to write this macro:
import macros, tables
type
Probability = range[0.0 .. 1.0]
ProbOne = range[1.0..1.0]
macro makePmf(name: untyped, tbl: untyped): untyped =
## Construct a Table[T, Probability] ensuring
## Sum(Probabilities) == 1.0
# helper templates
template asTable(tc: untyped): untyped =
tc.toTable
template asProb(f: float): untyped =
Probability(f)
# ensure that passed value is already is already
# a table constructor
tbl.expectKind nnkTableConstr
var
totprob: Probability = 0.0
fval: float
newtbl = newTree(nnkTableConstr)
# create Table[T, Probability]
for child in tbl:
child.expectKind nnkExprColonExpr
child[1].expectKind nnkFloatLit
fval = floatVal(child[1])
totprob += Probability(fval)
newtbl.add(newColonExpr(child[0], getAst(asProb(fval))))
# this serves as the check that probs sum to 1.0
discard ProbOne(totprob)
result = newStmtList(newConstStmt(name, getAst(asTable(newtbl))))
makePmf(uniformpmf, {"A": 0.25, "B": 0.25, "C": 0.25, "D": 0.25})
# this static block will show that the macro was evaluated at compile time
static:
echo uniformpmf
# the following invalid PMF won't compile
# makePmf(invalidpmf, {"A": 0.25, "B": 0.25, "C": 0.25, "D": 0.15})
Note: A cool benefit of using a macro is that nimsuggest (as integrated into VS Code) will even highlight attempts to create an invalid Pmf table.
Modula 3 has subrange types. (Subranges of ordinals.) So for your Example 1, if you're willing to map probability to an integer range of some precision, you could use this:
TYPE PROBABILITY = [0..100]
Add significant digits as necessary.
Ref: More about subrange ordinals here.
One of the arguments I've heard against functional languages is that single assignment coding is too hard, or at least significantly harder than "normal" programming.
But looking through my code, I realized that I really don't have many (any?) use patterns that can't be written just as well using single assignment form if you're writing in a reasonably modern language.
So what are the use cases for variables that vary within a single invocation of their scope? Bearing in mind that loop indexes, parameters, and other scope bound values that vary between invocations aren't multiple assignments in this case (unless you have to change them in the body for some reason), and assuming that you are writing in something a far enough above the assembly language level, where you can write things like
values.sum
or (in case sum isn't provided)
function collection.sum --> inject(zero, function (v,t) --> t+v )
and
x = if a > b then a else b
or
n = case s
/^\d*$/ : s.to_int
'' : 0
'*' : a.length
'?' : a.length.random
else fail "I don't know how many you want"
when you need to, and have list comprehensions, map/collect, and so forth available.
Do you find that you still want/need mutable variables in such an environment, and if so, what for?
To clarify, I'm not asking for a recitation of the objections to SSA form, but rather concrete examples where those objections would apply. I'm looking for bits of code that are clear and concise with mutable variables and couldn't be written so without them.
My favorite examples so far (and the best objection I expect to them):
Paul Johnson's Fisher-Yates algorithm answer, which is pretty strong when you include the big-O constraints. But then, as catulahoops points out, the big-O issue isn't tied to the SSA question, but rather to having mutable data types, and with that set aside the algorithm can be written rather clearly in SSA:
shuffle(Lst) ->
array:to_list(shuffle(array:from_list(Lst), erlang:length(Lst) - 1)).
shuffle(Array, 0) -> Array;
shuffle(Array, N) ->
K = random:uniform(N) - 1,
Ek = array:get(K, Array),
En = array:get(N, Array),
shuffle(array:set(K, En, array:set(N, Ek, Array)), N-1).
jpalecek's area of a polygon example:
def area(figure : List[Point]) : Float = {
if(figure.empty) return 0
val last = figure(0)
var first= figure(0)
val ret = 0
for (pt <- figure) {
ret+=crossprod(last - first, pt - first)
last = pt
}
ret
}
which might still be written something like:
def area(figure : List[Point]) : Float = {
if figure.length < 3
0
else
var a = figure(0)
var b = figure(1)
var c = figure(2)
if figure.length == 3
magnitude(crossproduct(b-a,c-a))
else
foldLeft((0,a,b))(figure.rest)) {
((t,a,b),c) => (t+area([a,b,c]),a,c)
}
Or, since some people object to the density of this formulation, it could be recast:
def area([]) = 0.0 # An empty figure has no area
def area([_]) = 0.0 # ...nor does a point
def area([_,_]) = 0.0 # ...or a line segment
def area([a,b,c]) = # The area of a triangle can be found directly
magnitude(crossproduct(b-a,c-a))
def area(figure) = # For larger figures, reduce to triangles and sum
as_triangles(figure).collect(area).sum
def as_triangles([]) = [] # No triangles without at least three points
def as_triangles([_]) = []
def as_triangles([_,_]) = []
def as_triangles([a,b,c | rest) = [[a,b,c] | as_triangles([a,c | rest])]
Princess's point about the difficulty of implementing O(1) queues with immutable structures is interesting (and may well provide the basis for a compelling example) but as stated it's fundamentally about the mutability of the data structure, and not directly about the multiple assignment issue.
I'm intrigued by the Sieve of Eratosthenes answer, but unconvinced. The proper big-O, pull as many primes as you'd like generator given in the paper he cited does not look easy to implement correctly with or without SSA.
Well, thanks everyone for trying. As most of the answers turned out to be either 1) based on mutable data structures, not on single-assignment, and 2) to the extent they were about single assignment form easily countered by practitioners skilled in the art, I'm going to strike the line from my talk and / or restructure (maybe have it in backup as a discussion topic in the unlikely event I run out of words before I run out of time).
Thanks again.
The hardest problem I've come across is shuffling a list. The Fisher-Yates algorithm (also sometimes known as the Knuth algorithm) involves iterating through the list swapping each item with a random other item. The algorithm is O(n), well known and long-since proven correct (an important property in some applications). But it requires mutable arrays.
That isn't to say you can't do shuffling in a functional program. Oleg Kiselyov has written about this. But if I understand him correctly, functional shuffling is O(n . log n) because it works by building a binary tree.
Of course, if I needed to write the Fisher-Yates algorithm in Haskell I'd just put it in the ST monad, which lets you wrap up an algorithm involving mutable arrays inside a nice pure function, like this:
-- | Implementation of the random swap algorithm for shuffling. Reads a list
-- into a mutable ST array, shuffles it in place, and reads out the result
-- as a list.
module Data.Shuffle (shuffle) where
import Control.Monad
import Control.Monad.ST
import Data.Array.ST
import Data.STRef
import System.Random
-- | Shuffle a value based on a random seed.
shuffle :: (RandomGen g) => g -> [a] -> [a]
shuffle _ [] = []
shuffle g xs =
runST $ do
sg <- newSTRef g
let n = length xs
v <- newListArray (1, n) xs
mapM_ (shuffle1 sg v) [1..n]
getElems v
-- Internal function to swap element i with a random element at or above it.
shuffle1 :: (RandomGen g) => STRef s g -> STArray s Int a -> Int -> ST s ()
shuffle1 sg v i = do
(_, n) <- getBounds v
r <- getRnd sg $ randomR (i, n)
when (r /= i) $ do
vi <- readArray v i
vr <- readArray v r
writeArray v i vr
writeArray v r vi
-- Internal function for using random numbers
getRnd :: (RandomGen g) => STRef s g -> (g -> (a, g)) -> ST s a
getRnd sg f = do
g1 <- readSTRef sg
let (v, g2) = f g1
writeSTRef sg g2
return v
If you want to make the academic argument, then of course it's not technically necessary to assign a variable more than once. The proof is that all code can be represented in SSA (Single Static Assignment) form. Indeed, that's the most useful form for many kinds of static and dynamic analysis.
At the same time, there are reasons we don't all write code in SSA form to begin with:
It usually takes more statements (or more lines of code) to write code this way. Brevity has value.
It's almost always less efficient. Yes I know you're talking about higher languages -- a fair scoping -- but even in the world of Java and C#, far away from assembly, speed matters. There are few applications where speed is irrelevant.
It's not as easy to understand. Although SSA is "simpler" in a mathematical sense, it's more abstract from common sense, which is what matters in real-world programming. If you have to be really smart to grok it, then it has no place in programming at large.
Even in your examples above, it's easy to poke holes. Take your case statement. What if there's an administrative option that determines whether '*' is allowed, and a separate one for whether '?' is allowed? Also, zero is not allowed for the integer case, unless the user has a system permission that allows it.
This is a more real-world example with branches and conditions. Could you write this as a single "statement?" If so, is your "statement" really different from many separate statements? If not, how many temporary write-only variables do you need? And is that situation significantly better than just having a single variable?
I've never identified such a case. And while you can always just invent new names, as in conversion to SSA form, I actually find it's easy and natural for each value to have its own name. A language like Haskell gives me a lot of choices about which values to name, and two different places to put name bindings (let and where). I find the single-assignment form quite natural and not at all difficult.
I do occasionally miss being able to have pointers to mutable objects on the heap. But these things have no names, so it's not the same objection. (And I also find that when I use mutable objects on the heap, I tend to write more bugs!)
I think you'll find the most productive languages allow you to mix functional and imperative styles, such as OCaml and F#.
In most cases, I can write code which is simply a long line of "map x to y, reduce y to z". In 95% of cases, functional programming simplifies my code, but there is one area where immutability shows its teeth:
The wide disparity between the ease of implementing and immutable stack and an immutable queue.
Stacks are easy and mesh well with persistence, queues are ridiculous.
The most common implementations of immutable queues use one or more internal stacks and stack rotations. The upside is that these queues run in O(1) most of the time, but some operations will run in O(n). If you're relying on persistence in your application, then its possible in principle that every operation runs in O(n). These queues are no good when you need realtime (or at least consistent) performance.
Chris Okasaki's provides an implementation of immutable queues in his book, they use laziness to achieve O(1) for all operations. Its a very clever, reasonably concise implementation of a realtime queue -- but it requires deep understanding of its underlying implementation details, and its still an order of magnitude more complex than an immutable stack.
In constrast, I can write a stack and queue using mutable linked lists which run in constant time for all operations, and the resulting code would be very straightforward.
Regarding the area of a polygon, its easy to convert it to functional form. Let's assume we have a Vector module like this:
module Vector =
type point =
{ x : float; y : float}
with
static member ( + ) ((p1 : point), (p2 : point)) =
{ x = p1.x + p2.x;
y = p1.y + p2.y;}
static member ( * ) ((p : point), (scalar : float)) =
{ x = p.x * scalar;
y = p.y * scalar;}
static member ( - ) ((p1 : point), (p2 : point)) =
{ x = p1.x - p2.x;
y = p1.y - p2.y;}
let empty = { x = 0.; y = 0.;}
let to_tuple2 (p : point) = (p.x, p.y)
let from_tuple2 (x, y) = { x = x; y = y;}
let crossproduct (p1 : point) (p2 : point) =
{ x = p1.x * p2.y; y = -p1.y * p2.x }
We can define our area function using a little bit of tuple magic:
let area (figure : point list) =
figure
|> Seq.map to_tuple2
|> Seq.fold
(fun (sum, (a, b)) (c, d) -> (sum + a*d - b*c, (c, d) ) )
(0., to_tuple2 (List.hd figure))
|> fun (sum, _) -> abs(sum) / 2.0
Or we can use the cross product instead
let area2 (figure : point list) =
figure
|> Seq.fold
(fun (acc, prev) cur -> (acc + (crossproduct prev cur), cur))
(empty, List.hd figure)
|> fun (acc, _) -> abs(acc.x + acc.y) / 2.0
I don't find either function unreadable.
That shuffle algorithm is trivial to implement using single assignment, in fact it's exactly the same as the imperative solution with the iteration rewritten to tail recursion. (Erlang because I can write it more quickly than Haskell.)
shuffle(Lst) ->
array:to_list(shuffle(array:from_list(Lst), erlang:length(Lst) - 1)).
shuffle(Array, 0) -> Array;
shuffle(Array, N) ->
K = random:uniform(N) - 1,
Ek = array:get(K, Array),
En = array:get(N, Array),
shuffle(array:set(K, En, array:set(N, Ek, Array)), N-1).
If the efficiency of those array operations is a concern, then that's a question about mutable data structures and has nothing to do with single assignment.
You won't get an answer to this question because no examples exist. It is only a question of familiarity with this style.
In response to Jason --
function forbidden_input?(s)
(s = '?' and not administration.qmark_ok) ||
(s = '*' and not administration.stat_ok) ||
(s = '0' and not 'root node visible' in system.permissions_for(current_user))
n = if forbidden_input?(s)
fail "'" + s + "' is not allowed."
else
case s
/^\d*$/ : s.to_int
'' : 0
'*' : a.length
'?' : a.length.random
else fail "I don't know how many you want"
I would miss assignments in a non-purely functional language. Mostly because they hinder the usefulness of loops. Examples (Scala):
def quant[A](x : List[A], q : A) = {
var tmp : A=0
for (el <- x) { tmp+= el; if(tmp > q) return el; }
// throw exception here, there is no prefix of the list with sum > q
}
This should compute the quantile of a list, note the accumulator tmp which is assigned to multiple times.
A similar example would be:
def area(figure : List[Point]) : Float = {
if(figure.empty) return 0
val last = figure(0)
var first= figure(0)
val ret = 0
for (pt <- figure) {
ret+=crossprod(last - first, pt - first)
last = pt
}
ret
}
Note mostly the last variable.
These examples could be rewritten using fold on a tuple to avoid multiple assignments, but that would really not help the readability.
Local (method) variables certainly never have to be assigned to twice. But even in functional programming re-assigning a variable is allowed. It's changing (part of) the value that's not allowed. And as dsimcha already answered, for very large structures (perhaps at the root of an application) it doesn't seem feasible to me to replace the entire structure. Think about it. The state of an application is all contained ultimately by the entrypoint method of your application. If absolutely no state can change without being replaced, you would have to restart your application with every keystroke. :(
If you have a function that builds a lazy list/tree then reduces it again, a functional compiler may be able to optimize it using deforestation.
If it's tricky, it might not. Then you're sort of out of luck, performance & memory wise, unless you can iterate and use a mutable variable.
Thanks to the Church-Turing Thesis, we know that anything that can be written in a Turing-complete language can be written in any Turing-complete language. So, when you get right down to it, there's nothing you can't do in Lisp that you couldn't do in C#, if you tried hard enough, or vice versa. (More to the point, either one is going to get compiled down to x86 machine language in most cases anyway.)
So, the answer to your question is: there are no such cases. All there are are cases that are easier for humans to comprehend in one paradigm/language or another-- and the ease of comprehension here is tied to training and experience.
Perhaps the main issue here is the style of looping in a language. In langauges where we use recursion, any values changing over the course of a loop are re-bound when the function is called again. Languages using iterators in blocks (e.g., Smalltalk's and Ruby's inject method) tend to be similar, though many people in Ruby would still use each and a mutable variable over inject.
When you code loops using while and for, on the other hand, you don't have the easy re-binding of variables that comes automatically when you can pass in several parameters to your chunk of code that does one iteration of the loop, so immutable variables would be rather more inconvenient.
Working in Haskell is a really good way to investigate the necessity of mutable variables, since the default is immutable but mutable ones are available (as IORefs, MVars, and so on). I've been recently, er, "investigating" in this way myself, and have come to the following conclusions.
In the vast majority of cases, mutable variables are not necessary, and I'm happy living without them.
For inter-thread communication, mutable variables are essential, for fairly obvious reasons. (This is specific to Haskell; runtime systems that use message passing at the lowest level don't need them, of course.) However, this use is rare enough that having to use functions to read and write them (readIORef fooRef val etc.) is not much of a burden.
I have used mutable variables within a single thread, because it seemed to make certain things easier, but later regretted it as I realized that it became very hard to reason about what was happening to the value stored there. (Several different functions were manipulating that value.) This was a bit of an eye-opener; in typical frog-in-the-pot-of-warming-water style, I'd not realized how easy Haskell had made it for me to reason about the use of values until I ran into an example of how I used to use them.
So these days I've come down fairly firmly on the side of immutable variables.
Since previous answers to this question have confused these things, I feel compelled to point out here quite forcefully that this issue is orthogonal to both purity and functional programming. I feel that Ruby, for example, would benefit from having single-assignment local variables, though possibly a few other changes to the language, such as adding tail recursion, would be necessary to make this truly convenient.
What about when you need to make small changes to large data structures? You don't really want to copy a whole array or large class every time you would modify a few elements.
I haven't really thought about this much except now that you're pointing it out.
Actually I try not using multiple assignments subconsciously.
Here's an example of what Im talking about, in python
start = self.offset%n
if start:
start = n-start
Written this way to avoid an unneccesary extra Modulo or subtraction. This is used with bignum style long ints, so its a worthwhile optimization. Thing about it, though, is that it really is a single assignment.
I wouldn't miss multiple assignment at all.
I know you asked for code that did show the benefits of mutable variables. And I wish I could provide it. But as pointed out before - there is no problem that can't be expressed in both fashions. And especially since you pointed out that jpalecek's area of a polygon example could be written with a folding algo (which is IMHO way messier and takes the problem to different level of complexity) - well it made me wonder why you are coming down on mutability so hard. So I'll try to make the argument for a common ground and an coexistence of immutable and mutable data.
In my opinion this question misses the point a bit. I know that us programmers are prone to liking things clean and simple but we sometimes miss that a mixture is possible as well. And that's probably why in the discussion about immutability there is seldom somebody taking the middle ground. I just wonder why, because let's face it - immutability is a great tool of abstracting all kinds of problems. But sometimes it is a huge pain in the ass. Sometimes it simply is too constraining. And that alone makes me stop and thing - do we really want to loose mutability? Is it really either-or? Isn't there some common ground we can arrive at? When does immutability help me achieve my goals faster, when does mutability? Which solution is easier to read and maintain? (Which for me is the biggest question)
A lot of these questions are influenced by a programmer's taste and by what they are used to program in. So I'll focus on one of the aspects that is usually the center of most pro-immutability arguments - Parallelism:
Often parallelism is thrown into the argument surrounding immutability. I've worked on problem sets that required 100+ CPUs to solve in a reasonable time. And it has taught me one very important thing: Most of the time parallelizing the manipulation of graphs of data is really not the kind of thing that will be the most efficient way to parallelize. It sure can benefit greatly, but imbalance is a real problem in that problem-space. So usually working on multiple mutable graphs in parallel and exchanging information with immutable messages is way more efficient. Which means, when I know that the graph is isolated, that I have not revealed it to the outside world, I would like to perform my operations on it in the most concise manner I can think of. And that usually involves mutating the data. But after these operation on the data I want to open the data up to the whole world - and that's the point where I usually get a bit nervous, if the data is mutable. Because other parts of the program could corrupt the data, the state becomes invalid, ... because after opening up to the world the data often does get into the world of parallelism.
So real world parallel programs usually have areas where data graphs are used in definitive single thread operations - because they simply are not known to the outside - and areas where they could be involved in multi-threaded operations (hopefully just supplying data not being manipulated). During those multi-threaded parts we never want them to change - it simply is better to work on outdated data than on inconsistent data. Which can be guaranteed by the notion of immutability.
That made me come to a simple conclusion: The real problem for me is that non of the programming languages I am familiar with allow me to say: "After this point this whole data structure shal be immutable" and "give me a mutable copy of this immutable data structure here, please verify that only I can see the mutable copy". Right now I have to guarantee it myself by flipping a readonly bit or something similar. If we could have compiler support for it, not only would it guarantee for me that I did not do anything stupid after flipping said bit, but it could actually help the compiler do various optimizations that it couldn't do before. Plus - the language would still be attractive for programmers that are more familiar with the imperative programming model.
So to sum up. IMHO programs usually have a good reason to use both immutable and mutable representations of data graphs. I would argue that data should be immutable by default and the compiler should enforce that - but we should have the notion of private mutable representations, because there naturally are areas where multi-threading will never reach - and readability and maintainability could benefit from a more imperative structuring.