I have the following haskell code:
Why doesn't x1's pattern matching to function f?
It's pretty hard to read as-is. Let's use some creative whitespace to line things up.
f ( [_ ]:[(x,[xs ])]:[y ,ys ] :[]) = 1
x1 = [[(1,[1,2])],[(1,[1,2])],[(1,[1,2]),(1,[1,2])],[]]
Okay. So there's actually a couple different things that aren't going as you expect!
[xs] does not match [1, 2], because [xs] is a one-element list and [1, 2] is a two-element list (possible fix: xs instead of [xs])
[y, ys] happens to match, but I suspect not in the way you intended: y matches to the first element of the list, just as I think you intend, but ys to the second element of the list, not the remainder of the list I think you intend (possible fix: (y:ys) instead of [y, ys])
your pattern's :[] matches the closing bracket of a list definition, not a final [] element (possible fix: :[]:[] instead of :[]; the first [] there matches the element, and the second [] matches the end-of-list marker)
Please excuse the beginner question. I couldn't find an appropriate answer in any Mathematica tutorial.
I am confused why a definition as a function or a definition in terms of a simple replacement produce different results. Consider this example (Mathematica 9 code):
In[397]:= ClearAll["Global`*"]
In[398]:= Test := 3 c^2 + d^4
In[399]:= v[f_] := D[f, c]
In[400]:= v[Test]
Out[400]= 6 c
The first definition of this simple derivative function "v" acting on a variable is fine. Defining a replacement Test = ... to replace the variable produces the expected result (It derives 3c^2+d^4 with respect to c and answers 6c).
However if I define a function instead of a simple replacement this does not work:
In[401]:= TestFunction[a_, b_] := 3 a^2 + b^4
In[403]:= vFunction[f_[a_, b_]] := D[f[a, b], a]
In[405]:= vFunction[TestFunction[a, b]]
Out[405]= \!\(
\*SubscriptBox[\(\[PartialD]\), \(3\
\*SuperscriptBox[\(a\), \(2\)]\)]\((3\
\*SuperscriptBox[\(a\), \(2\)] +
\*SuperscriptBox[\(b\), \(4\)])\)\)
Why is that? I am risking to look like a moron here, but please enlighten me!
For your convenience, I uploaded a copy of my workbook here
Thanks a lot,
Michael
Do this instead
vFunction[f_,a_,b_]:=D[f[a,b],a];
and when you need derivatives simply use vFunction[TestFunction,a,b] to get it.
When you write down f[x], it means the evaluated value of f with argument value x. So, f[x] is technically not a function anymore. What you want as the argument of vFunction[] is the function TestFunction, not the evaluated value.
The usage message for Set reminds us that multiple assignments can easily be made across two lists, without having to rip anything apart. For example:
Remove[x1, x2, y1, y2, z1, z2];
{x1, x2} = {a, b}
Performs the assignment and returns:
{a, b}
Thread, commonly used to generate lists of rules, can also be called explicitly to achieve the same outcome:
Thread[{y1, y2} = {a, b}]
Thread[{z1, z2} -> {a, b}]
Gives:
{a, b}
{z1 -> a, z2 -> b}
However, employing this approach to generate localized constants generates an error. Consider this trivial example function:
Remove[f];
f[x_] :=
With[{{x1, x2} = {a, b}},
x + x1 + x2
]
f[z]
Here the error message:
With::lvset: "Local variable specification {{x1,x2}={a,b}} contains
{x1,x2}={a,b}, which is an assignment to {x1,x2}; only assignments
to symbols are allowed."
The error message documentation (ref/message/With/lvw), says in the 'More Information' section that, "This message is generated when the first element in With is not a list of assignments to symbols." Given this explanation, I understand the mechanics of why my assignment failed. Nonetheless, I'm puzzled and wondering if this is necessary restriction by WRI, or a minor design oversight that should be reported.
So here's my question:
Can anyone shed some light on this behavior and/or offer a workaround? I experimented with trying to force Evaluation, without luck, and I'm not sure what else to try.
What you request is tricky. This is a job for macros, as already exposed by the others. I will explore a different possibility - to use the same symbols but put some wrappers around the code you want to write. The advantage of this technique is that the code is transformed "lexically" and at "compile-time", rather than at run-time (as in the other answers). This is generally both faster and easier to debug.
So, here is a function which would transform the With with your proposed syntax:
Clear[expandWith];
expandWith[heldCode_Hold] :=
Module[{with},
heldCode /. With -> with //. {
HoldPattern[with[{{} = {}, rest___}, body_]] :>
with[{rest}, body],
HoldPattern[
with[{
Set[{var_Symbol, otherVars___Symbol}, {val_, otherVals___}], rest___},
body_]] :>
with[{{otherVars} = {otherVals}, var = val, rest}, body]
} /. with -> With]
Note that this operates on held code. This has the advantage that we don't have to worry about possible evaluation o the code neither at the start nor when expandWith is finished. Here is how it works:
In[46]:= expandWith#Hold[With[{{x1,x2,x3}={a,b,c}},x+x1+x2+x3]]
Out[46]= Hold[With[{x3=c,x2=b,x1=a},x+x1+x2+x3]]
This is, however, not very convenient to use. Here is a convenience function to simplify this:
ew = Function[code, ReleaseHold#expandWith#Hold#code, HoldAll]
We can use it now as:
In[47]:= ew#With[{{x1,x2}={a,b}},x+x1+x2]
Out[47]= a+b+x
So, to make the expansion happen in the code, simply wrap ew around it. Here is your case for the function's definition:
Remove[f];
ew[f[x_] := With[{{x1, x2} = {a, b}}, x + x1 + x2]]
We now check and see that what we get is an expanded definition:
?f
Global`f
f[x_]:=With[{x2=b,x1=a},x+x1+x2]
The advantage of this approach is that you can wrap ew around an arbitrarily large chunk of your code. What happens is that first, expanded code is generated from it, as if you would write it yourself, and then that code gets executed. For the case of function's definitions, like f above, we cansay that the code generation happens at "compile-time", so you avoid any run-time overhead when usin the function later, which may be substantial if the function is called often.
Another advantage of this approach is its composability: you can come up with many syntax extensions, and for each of them write a function similar to ew. Then, provided that these custom code-transforming functions don't conlict with each other, you can simply compose (nest) them, to get a cumulative effect. In a sense, in this way you create a custom code generator which generates valid Mathematica code from some Mathematica expressions representing programs in your custom languuage, that you may create within Mathematica using these means.
EDIT
In writing expandWith, I used iterative rule application to avoid dealing with evaluation control, which can be a mess. However, for those interested, here is a version which does some explicit work with unevaluated pieces of code.
Clear[expandWithAlt];
expandWithAlt[heldCode_Hold] :=
Module[{myHold},
SetAttributes[myHold, HoldAll];
heldCode //. HoldPattern[With[{Set[{vars__}, {vals__}]}, body_]] :>
With[{eval =
(Thread[Unevaluated[Hold[vars] = Hold[vals]], Hold] /.
Hold[decl___] :> myHold[With[{decl}, body]])},
eval /; True] //. myHold[x_] :> x]
I find it considerably more complicated than the first one though.
The tricky issue is to keep the first argument of Set unevaluated.
Here is my suggestion (open to improvements of course):
SetAttributes[myWith, HoldAll];
myWith[{s : Set[a_List, b_List]}, body_] :=
ReleaseHold#
Hold[With][
Table[Hold[Set][Extract[Hold[s], {1, 1, i}, Hold],
Extract[Hold[s], {1, 2, i}]], {i, Length#b}], Hold#body]
x1 = 12;
Remove[f];
f[x_] := myWith[{{x1, x2} = {a, b}}, x + x1 + x2]
f[z]
results in
a+b+z
Inspired by halirutan below I think his solution, made slightly more safely, is equivalent to the above:
SetAttributes[myWith, HoldAll];
myWith[{Set[a : {__Symbol}, b_List]} /; Length[a] == Length[b],
body_] :=
ReleaseHold#
Hold[With][
Replace[Thread[Hold[a, b]], Hold[x_, y_] :> Hold[Set[x, y]], 1],
Hold#body]
The tutorial "LocalConstants" says
The way With[{x=Subscript[x, 0],...},body] works is to take body, and replace every
occurrence of x, etc. in it by Subscript[x, 0], etc. You can think of With as a
generalization of the /. operator, suitable for application to Mathematica code instead of
other expressions.
Referring to this explanation it seems obvious that something like
x + x1 + x2 /. {x1, x2} -> {a, b}
will not work as it might be expected in the With notation.
Let's assume you really want to hack around this. With[] has the attribute HoldAll, therefore everything you give as first parameter is not evaluated. To make such a vector-assignment work you would have to create
With[{x1=a, x2=b}, ...]
from the vector-notation. Unfortunately,
Thread[{a, b} = {1, 2}]
does not work because the argument to Thread is not held and the assignment is evaluated before Thread can do anything.
Lets fix this
SetAttributes[myThread, HoldFirst];
myThread[Set[a_, b_]] := mySet ### Transpose[{a, b}]
gives
In[31]:= myThread[{a, b, c} = {1, 2, 3}]
Out[31]= {mySet[a, 1], mySet[b, 2], mySet[c, 3]}
What looks promising at first, just moved the problem a bit away. To use this in With[] you have to replace at some point the mySet with the real Set. Exactly then, With[] does not see the list {a=1, b=2, c=3} but, since it has to be evaluated, the result of all assignments
In[32]:= With[
Evaluate[myThread[{a, b, c} = {1, 2, 3}] /. mySet :> Set], a + b + c]
During evaluation of In[32]:= With::lvw: Local
variable specification {1,2,3} contains 1, which is not an assignment to a symbol. >>
Out[32]= With[{1, 2, 3}, a + b + c]
There seems to be not easy way around this and there is a second question here: If there is a way around this restriction, is it as fast as With would be or do we lose the speed advantage compared to Module? And if speed is not so important, why not using Module or Block in the first place?
You could use Transpose to shorten Rolfs solution by 100 characters:
SetAttributes[myWith, HoldAll];
myWith[{Set[a_List, b_List]}, body_] :=
ReleaseHold[Hold[With][Hold[Set[#1, #2]] & ### Transpose[{a, b}],
Hold#body
]]
#Heike, yep the above breaks if either variable has already a value. What about this:
SetAttributes[myWith, HoldAll];
myWith[{Set[a_List, b_List]}, body_] :=
ReleaseHold#
Hold[With][Thread[Hold[a, b]] /. Hold[p__] :> Hold[Set[p]],
Hold#body]
there! I'm building an image-processing application in swing/clojure, and right now I need to develop an image panel in which I can click and compute data. Thanks to coobird, I now have a good idea on how to do it in Java, but I still don't get many issues on its integration with Clojure.
Let's take a look at how coobird suggested me doing. First, we should extend a class in Java. In clojure, we do this with the proxy macro, so we'd have something like this:
(def painting-panel
(proxy [JPanel] []))
The next step is to create the class constructor and set some variables.
I can define functions after the second argument of proxy, but how can I create the constructor? Is painting-panel the name of this class (therefore the name of the function I should create)?
How can I deal with class variables? Should I define them with a let, like I did?
Are this and super available for me to use, like I did below?
(def painting-panel
(let [background-image (Image.)
point-clicked (Point.)]
(proxy [JPanel] []
(paintComponent [g]
(do ((.paintComponent super) g)
(doto g
(.drawImage background-image 0 0 nil)
(.fillRect (.x point-clicked) (.y point-clicked) 1 1))))
(painting-panel []; constructor?
((.addMouseListener this)
(proxy [MouseAdapter] []
(mouseClicked [e]
(do
(def point-clicked (.getPoint e))
(.repaint this)))))))))
Suggestions and code corrections are also welcome!
Thank you!
proxy actually creates an instance tada! You don't need to create a constructor.
Yes, but consider using a clojure ref instead. Also using def like that on the second last line is nasty! it creates a global binding for point-clicked when your logic relies on the lexically scoped one created by let.
(proxy-super paintComponent g), and yes "this" is available
This works for me:
(let [click (ref nil)
panel (proxy [javax.swing.JPanel] []
(paintComponent [g]
(proxy-super paintComponent g)
(.drawImage g (.getImage
(javax.swing.ImageIcon. "play.png"))
0 0 (.getWidth this) (.getHeight this) nil)
(if #click
(.fillRect g (:x #click) (:y #click) 10 10))))]
(.addMouseListener panel
(proxy [java.awt.event.MouseAdapter] []
(mouseClicked [e]
(let [p (.getPoint e)]
(dosync (ref-set click {:x (.x p), :y (.y p)})))
(javax.swing.SwingUtilities/invokeLater #(.repaint panel)))))
(doto (javax.swing.JFrame.)
(.setContentPane panel)
(.setSize 200 200)
(.show)))