sml

问题 I'm using SML/NJ 110.79, which includes support for new structures defined by the Successor ML project. Among others, the Fn structure. As it happens, I already had an identically named structure in one of my personal project with utilities, which worked fine before 110.79. With 110.79, for this .cm file: group is $/basis.cm $SMACKAGE/sml-extras/v0.1.0/sources.sml.cm I get the following error, though: sources.cm:3.3-3.45 Error: structure Fn imported from $SMLNJ-BASIS/(basis.cm):basis-common

问题 I need to implement a function ziprev : 'a list -> 'b list -> ('a * 'b) list - ziprev [1,2,3,4] [10,20,30,40]; val it = [(1,40),(2,30),(3,20),(4,10)] : (int * int) list Using a function that I already created: - zipW (fn (x, y) => x + y) [1,2,3,4] [10,20,30,40]; val it = [11,22,33,44] : int list and the List.rev from the library. I have no idea how to do a function with two libraries. Any suggestions? 回答1: Hint 1: Compare the result of your ziprev with List.zip [1,2,3,4] [10,20,30,40] You

问题 I am new to SML, and have asked people about the error that I got. But, I can't find where the problem is. The error message is received is: stdIn:10.1-16.48 Error: unresolved flex record (need to know the names of ALL the fields in this context) type: {1:''Y, 2:''X; ''Z} I have two functions. The first function is reverse, which is to reverse a list and return it. For example, reversing [1,2,3,4] to [4,3,2,1]. This function has absolutely no problems. fun reverse(L) = if L = nil then nil

问题 In SML, I've been taught the idiomatic way to define a variable local to the function as: fun correct_fun() = let val x = 1 in x + 2 end Why do I have to use let , and not just val like so: fun incorrect_fun() = val x = 1 x + 2 The incorrect_fun() throws an error but I don't understand why. Why can't val be used inside a function without let ? 回答1: Why can't val be used inside a function without let ? Because val is a kind of declaration, let is a kind of expression, and a function body is an

问题 fun a(list) = let val num = length(hd(list)) fun inner(list) = if num = length(hd(list)) then if tl(list) = nil then true else inner(tl(list)) else false in if length(hd(list))-1 = length(tl(list)) then inner(tl(list)) else false end; this is ml code and I got this warning and type. stdIn:6.16 Warning: calling polyEqual val a = fn : ''a list list -> bool I don't understand about the warning. why it appear and the type. ''a why it has two '? ''? what is the difference between 'a list list and

问题 How to seed SML/NJ's random number generator on a Windows machine? The function Random.rand() takes a pair of integers and uses them to seed the random number generator. Based on my experience with other porgramming languages, I would expect there to be a relatively easy way to seed it based on the system clock (something like srand(time(null)); in C). Unless I am overlooking something obvious, there doesn't seem to be any straightforward way, at least if you are using Windows. The closest I

问题 Could someone please explain the concept of currying to me. I am primarily learning it because we are learning ML in my 'modern programming language' class for a functional language introduction. In particular you can use this example: -fun g a = fn b => a+b; val g = fn: int -> int -> int -g 2 3; val it = 5 : int I'm confused how these parameters are passed or how to even think about it in the first place. Thank you for any help. 回答1: In this case, you make the currying explicit, so it should

问题 In a learning environment, what are my options to provide type signatures for functions? Standard ML doesn't have top-level type signatures like Haskell. Here are the alternatives I have considered: Module signatures, which require either a separate signature file, or the type signature being defined in a separate block inside the same file as the module itself. This requires the use of modules, and in any production system that would be a sane choice. Modules may seem a little verbose in a

问题 I have the following two datatypes: datatype leaf = Slist of string list | Real of real | nil; datatype 'a tree = Empty | Node of leaf * 'a tree * 'a tree * 'a tree; The code below goes through all trees of length one/two and forms a list of the values in the leafs. fun list12(Empty:'a tree) = nil | list12(Node(leaf leaf1, 'a tree a1, 'a tree a2, 'a tree a3)) = if (not(a1 = Empty) andalso not(a2 = Empty) andalso not(a3 = Empty)) then list12(a1)::list12(a2)::list12(a3) else leaf1::list12(a1):

问题 I was wondering if it is possible to call a python file in an SML program, and if so how can you do it? I have tried researching how to do this, but have only found documentation on how to call other SML files. 回答1: I think OS.Process.system "python myscript.py" should work. See: http://sml-family.org/Basis/os-process.html 来源： https://stackoverflow.com/questions/43353918/call-a-python-file-in-a-sml-program