I don't know why my code doesn't work.
fun lookup _ [] = 0
| lookup key ((k,v)::entries) =
if k = key
then v
else (lookup key entries)
That's what happened when I tested it in cmd.
val lookup = fn : ''a -> (''a * int) list -> int
- lookup (1,[(1,2),(2,3)]);
val it = fn : ((int * (int * int) list) * int) list -> int
There's nothing wrong with your code, you just didn't call lookup with enough arguments. You make a common mistakes among beginner SML programmers coming from other languages. I'll try to clarify that.
First, the most important thing to know about functions in Standard ML is this:
All functions in Standard ML take exactly one argument.
You might be confused at this point, because your lookup function looks as if it's taking two arguments. It kind of does, but not really.
There are two main "workarounds" (I'm using quotes because this is actually a great feature of the language) for representing functions that take multiple arguments:
1. Using curried functions
If you need to write a function which, conceptually, needs three arguments, then you'd write this:
fun quux a =
fn b =>
fn c =>
(* do something with a, b and c *)
So, quux is:
a function, which takes an argument a and returns
a function, which takes an argument b and returns
a function, which takes an argument c and returns
the result computed using a, b and c
How would you call quux? Like this, right?
((quux a) b) c
But function application is already left associative, so we can actually write this:
quux a b c
We don't need parentheses to "call" functions! In Standard ML parentheses don't mean "call this function". They're used just for grouping expressions together when you want to change associativity, like in mathematics: (1 + 2) * 3.
Because defining quux as above is really cumbersome, there's a syntactic shortcut in the language. Instead of writing this:
fun quux a =
fn b =>
fn c =>
(* do something with a, b and c *)
We can write just this:
fun quux a b c = (* do something with a, b and c *)
But, they're the same thing. quux is still a function which takes just argument a and returns a new function with argument b, which returns a new function which argument c.
Ok, so that was one way of representing multi-argument functions in Standard ML. It's also the one you used to define lookup.
2. Using tuples
Another common way of representing multi-argument functions is to accept a tuple (which may have from 2 to as many components as you wish). Here's the above example using a tuple now:
fun quux args =
case args of
(a,b,c) => (* do something with a, b and c *)
How could we call quux now? Like this:
quux (a,b,c)
Notice that I put a space between quux and the tuple. It's optional, but I do it all the time to keep remembering that function application in standard ML is not represented by parentheses. quux gets called because it's been put before the tuple (a,b,c). Tuples, however, do require parentheses, which is why you're seeing them above.
Again, as before, it's cumbersome to define quux like this:
fun quux args =
case args of
(a,b,c) => (* do something with a, b and c *)
So we can actually use another great feature of the language, pattern matching in argument position, that lets us write this:
fun quux (a,b,c) = (* do something with a, b and c *)
Ok, now we can really answer your question.
You defined lookup using the curried function syntax:
fun lookup _ [] = 0
But you "called" lookup using the tuple syntax, where 1 is the first element of the tuple and [(1,2),(2,3)] is the second element.
lookup (1, [(1,2),(2,3)])
Why doesn't the compiler complain, though. In this unfortunate case, it doesn't because it happens that the type of the first argument of lookup is a tuple. So, you've basically called lookup with a single argument.
What you wanted was this:
lookup 1 [(1,2),(2,3)]
Notice that I'm not defining a tuple anymore.
Related
type card = int
type game = { dimension : int; p1 : card list; }
let fn (_dimension : int) (p1 : card list) : bool =
(int)p1 = (int)_dimension * 2
I want to check that p1 is exactly twice the size of dimension.
Your code doesn't look very much like OCaml code, so it's difficult to know how to help :-)
There is no operation in OCaml to change the type of a value. That wouldn't make sense in a strongly typed language. There are some things you can do with types, but "casting" isn't one of them.
So, there is no valid expression that looks like (int) expr. As #glennsl points out, there is a function that returns the length of a list. If that's what you're trying to calculate, you can use List.length _dimension. The other occurrences of (int) you can just remove. They aren't valid OCaml.
Your definition of fn doesn't use the type game anywhere, so the definition of game is unnecessary. However it makes me worry that you're expecting the definition to have an effect.
If you leave out all the type ascriptions in your definition of fn the compiler will deduce the most general type for your function. This means you can call it with any list to check its length. That would be more idiomatic in OCaml. I.e., you don't need to specify that p1 is a list of cards. The function makes sense for any list.
smlnj will make overloaded operator, like op + to use int by default, now I want to it returns a function in real * real -> real, how can I do in inline way?
"inline way" means not something like binding a new val:
fun add(x:real,y:real) = x + y;
If my memory is correct there is some grammar allows sml it to just do something like "cast" op + to real, but I can't really find it anywhere..
There are various ways that you can get SML to type op+ as the real counterpart.
Depending on what ever code you have,
You can as suggested, type annotate the surrounding function, thus enforcing the parameters to op+ to be of type real.
Since you are nonfixing the addition function (presumably for use as a higher order function?), you could just as well pass along the addition function from the real module Real.+
Or you could annotate it like this: op+ : real * real -> real, which is really ugly and stupid, considering you can use Real.+ instead. But it is an option.
If the default instance of the operator is not the one you need for your value's type, you can use type annotation on the operands to enforce the desired typing.
For example, while
val f = fn a => a + a
will be typed int -> int, this value
val g = fn a:real => a + a
will be typed real -> real.
You could declare
open Real
in the scope where you define the function, but I strongly advise against that. Type-annotating the function is the best way. You don't have to annotate every parameter, btw, it's enough to do one, or in this case, even the return type:
fun add(x : real, y) = x + y
fun add(x, y) : real = x + y
type foo = A of int * int | B of (int * int)
What is the difference between int * int and (int * int) there? The only difference I see is in pattern matching:
let test_foo = function
| A (f, s) -> (f, s)
| B b -> b
Is it just a syntactic sugar? How do you select which one to use? Is there any performance difference between these two forms?
Yes, there is a performance difference:
In memory A (23, 42) will contain a tag identifying it as an A and the two integers 23 and 42. B (23, 42) will contain a tag identifying it as a B and a pointer to a tuple containing the integers 23 and 42. So there will be one additional memory allocation when creating a B and one additional level of indirection when accessing the individual values inside a B. So in cases where you don't actually use the constructor arguments as a tuple, using A will involve less overhead than using B.
On the other hand your test_foo function will create a new tuple every time it is called with an A value, but when it is called with a B value it will simply return the tuple that already exists in memory. So test_foo is a cheaper operation for B than it is for A. So if you'll be using the constructor's arguments as a tuple and you will do so multiple times for the same value, using B will be cheaper.
So if you're going to be using the constructor arguments as a tuple, it makes sense to use a constructor taking a tuple because you can get at the tuple using pattern matching with less code and because it will avoid having to create tuples from the same value multiple times. In all other cases not using a tuple is preferable because it involves less memory allocation and less indirection.
As already said, the constructor of A takes two int, whereas the constructor of B takes an ordered pair.
so you can write
let bar = A (1, 2)
or
let bar = B (1, 2)
or
let bar = (1, 2)
let baz = B bar
but you cannot write
let bar = (1, 2)
let baz = A bar
Moreover, in your pattern matching, you can still match the content of B as two int, but you cannot match the content of A as value bound to an ordered pair
let test_foo = function
| A a -> a (* wrong *)
| B (f, s) -> (f, s) (* ok *)
They are two different types. The interpretation of this syntax is ambiguous at the * operator. It may be reduced into the form:
type x = Y * Z in which the '*' is associated with the type keyword in OCaml
or
int * int in which the * is used in the capacity of an operator that constructs a tuple
The default precedence takes it to the former. By putting a parenthesis around the (int * int) you override the default precedence and force the latter interpretation.
This is one of the tricky things in OCaml syntax -- even though it looks like you are declaring a constructor with a tuple data type (A of int * int), and even though when you use the constructor, it looks like you are giving a tuple to it (A (2,3)), that is not actually what is happening.
If you actually construct a tuple value and try to pass it to the constructor, it will not compile -- let x = (2,3) in A x. Rather, the * in the constructor definition and the (,) in the constructor use expression are simply the syntax for a constructor of multiple arguments. The syntax imitates that of a constructor with a tuple argument, but is actually separate. The extra parentheses are necessary if you want to actually make a constructor with a single tuple argument.
Is there any way to check/test the type of a variable?
I want to use it like this:
if x = int then foo
else if x = real then bar
else if x = string then ...
else .....
ML languages are statically typed, so it's not possible for something to have different types at different times. x can't sometimes have type int and at other times have the type string. If you need behavior like this, the normal way to go about it is to wrap the value in a container that encodes type information, like:
datatype wrapper = Int of int | Real of real | String of string
Then you can pattern-match on the constructor:
case x of Int x -> foo
| Real x -> bar
| String x -> ...
In this case, x is clearly typed as a wrapper, so that will work.
It's not possible to do what you want in general, even if x is of polymorphic type (without doing the wrapping yourself as Chuck suggests).
This is a deliberate design decision; it makes it possible to make very strong conclusions about functions, just based on their types, that you couldn't make otherwise. For instance, it lets you say that a function with type 'a -> 'a must be the identity function (or a function that always throws an exception, or a function that never returns). If you could inspect what 'a was at runtime, you could write a sneaky program like
fun sneaky (x : 'a) : 'a = if x = int then infinite_loop() else x
that would violate the rule. (This is a pretty trivial example, but there are lots of less-trivial things you can do by knowing your type system has this property.)
Basically, I want to have a function to return a polymorphic function, some thing like this:
fun foo () = fn x => x
So the foo function takes in a value of type unit and returns a polymorphic identity function
and the compiler is happy with that, it gives me:
val foo = fn : unit -> 'a -> 'a
but once I actually call the foo function, the return value is not what I expected
val it = fn : ?.X1 -> ?.X2
Can't generalize because of value restriction it says, any help? thanks in advance
For technical reasons, you are not allowed to generalize (i.e., make polymorphic) the results of a function call. The result of a call must have a monomorphic type. If this weren't the case, you could subvert the type system by the following dirty trick:
Call ref [] and get back a list of type forall 'a . 'a list ref
Insert a string.
Remove a function
and there you are: you are now executing the contents of an arbitrary string as code. Not Good.
By insisting that the value returned by ref [] be monomorphic, you ensure that it can be used as a list of strings or a list of functions but not both. So this is part of the price we pay for type safety.