神奇的sprintf函数-如何包装? [英] Magic sprintf function - how to wrap it?
问题描述
我正在尝试包装对sprintf
函数的调用.这是我的尝试:
I am trying to wrap a call to sprintf
function. Here's my attempt:
let p format args = "That was: " + (sprintf format args)
let a = "a"
let b = "b"
let z1 = p "A %s has invalid b" a
这似乎可行,输出为
val p : format:Printf.StringFormat<('a -> string)> -> args:'a -> string
val a : string = "a"
val b : string = "b"
val z1 : string = "That was: A a has invalid b"
但是它不能用于多个arg:
But it wouldn't work with more than one arg:
let z2 = p "A %s has invalid b %A" a b
我收到编译时错误:
let z2 = p "A %s has invalid b %A" a b;;
---------^^^^^^^^^^^^^^^^^^^^^^^^^^^
stdin(7,10): error FS0003: This value is not a function and cannot be applied
我如何创建一个可以与任意数量的args一起使用的函数?
How can I create a single function which would work with any number of args?
UPD . Tomas建议使用
UPD. Tomas has suggested to use
let p format = Printf.kprintf (fun s -> "This was: " + s) format
它确实有效.这是一个例子
It works indeed. Here's an example
let p format = Printf.kprintf (fun s -> "This was: " + s) format
let a = p "something like %d" 123
// val p : format:Printf.StringFormat<'a,string> -> 'a
// val a : string = "This was: something like 123"
但是问题是我的功能的主要目的是做一些除格式化工作之外的工作,因此我尝试使用建议的代码,如下所示
But the thing is that main purpose of my function is to do some work except for formatring, so I tried to use the suggested code as follows
let q format =
let z = p format // p is defined as suggested
printf z // Some work with formatted string
let z = q "something like %d" 123
它不再起作用:
let z = q "something like %d" 123;;
----------^^^^^^^^^^^^^^^^^^^
stdin(30,15): error FS0001: The type ''c -> string' is not compatible with the type 'Printf.TextWriterFormat<('a -> 'b)>'
我该如何解决?
推荐答案
为此,您需要使用currying-函数p
需要采用format
并返回由printf
函数(然后可以是带有一个或多个参数的函数).
For this to work, you need to use currying - your function p
needs to take the format
and return a function returned by one of the printf
functions (which can then be a function taking one or more arguments).
这不能使用sprintf
来完成(因为您必须显式传播参数.但是,您可以使用kprintf
,它将 continuation 作为第一个参数:: >
This cannot be done using sprintf
(because then you would have to propagate the arguments explicitly. However, you can use kprintf
which takes a continuation as the first argument::
let p format = Printf.kprintf (fun s -> "This was: " + s) format
以格式化的字符串调用延续,因此您可以在返回之前对结果字符串进行任何所需的操作.
The continuation is called with the formatted string and so you can do whatever you need with the resulting string before returning.
编辑:要回答您的扩展问题,诀窍是将所有其他工作放到续篇中:
To answer your extended question, the trick is to put all the additional work into the continuation:
let q format =
let cont z =
// Some work with formatted string
printf "%s" z
Printf.kprintf cont format
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