为什么有些FAKE方法在目标函数内不起作用？

Question

考虑以下:

#r @"FakeLib.dll"

open Fake
open Fake.StringHelper
open Fake.ProcessHelper

Shell.Exec("mkdir","exampleDirectory")

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
)

Target "Clean" ( fun () -> 
    trace  "Cleaning..."
)

Target "Deploy" (fun () -> 
    trace  "Deploying..."
)

"DoStuff"
    ==>"Clean"
    ==>"Deploy"

RunTargetOrDefault "Deploy"

上面的脚本工作正常,但当我在目标中移动Shell.Exec时,如下所示:

#r @"FakeLib.dll"

open Fake
open Fake.StringHelper
open Fake.ProcessHelper

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    Shell.Exec("mkdir","exampleDirectory")
)

Target "Clean" ( fun () -> 
    trace  "Cleaning..."
)

Target "Deploy" (fun () -> 
    trace  "Deploying..."
)

"DoStuff"
    ==>"Clean"
    ==>"Deploy"

RunTargetOrDefault "Deploy"

我最后得到一个错误:

FsiEvaluationException:

Error:

        DeployScript.fsx(10,5): error FS0001: This expression was expected to have type
            unit
        but here has type
            int


Output: [Loading C:\Users\myusername\Desktop\SomeFolder\DeployScript.fsx]


Input: C:\Users\myusername\Desktop\SomeFolder\DeployScript.fsx
\Arguments:
  C:\fsi.exe

Exception: Yaaf.FSharp.Scripting.FsiEvaluationException: Error while compiling or executing fsharp snippet. ---> System.Exception: Operation failed. The error text has been print the error stream. To return the corresponding FSharpErrorInfo use the EvalInteractionNonThrowing, EvalScriptNonThrowing or EvalExpressionNonThrowing
   at Microsoft.FSharp.Compiler.Interactive.Shell.FsiEvaluationSession.commitResult[a,b](FSharpChoice`2 res)
   at Microsoft.FSharp.Compiler.Interactive.Shell.FsiEvaluationSession.EvalScript(String filePath)
   at Yaaf.FSharp.Scripting.Helper.evalScript@1303.Invoke(String arg00) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1303
   at Yaaf.FSharp.Scripting.Helper.save_@1276-2.Invoke(Unit unitVar0) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1277
   at Yaaf.FSharp.Scripting.Helper.consoleCapture[a](TextWriter out, TextWriter err, FSharpFunc`2 f) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1221
   at Yaaf.FSharp.Scripting.Helper.redirectOut@1247[a](Boolean preventStdOut, OutStreamHelper out, OutStreamHelper err, FSharpFunc`2 f) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1254
   at Yaaf.FSharp.Scripting.Helper.save_@1275-1.Invoke(String text) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1276
   --- End of inner exception stack trace ---
   at Yaaf.FSharp.Scripting.Helper.save_@1275-1.Invoke(String text) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1284
   at Yaaf.FSharp.Scripting.Helper.session@1306.Yaaf-FSharp-Scripting-IFsiSession-EvalScriptWithOutput(String ) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1308
   at Fake.FSIHelper.runScriptUncached(Boolean useCache, String scriptPath, IEnumerable`1 fsiOptions, Boolean printDetails, CacheInfo cacheInfo, TextWriter out, TextWriter err) in C:\code\fake\src\app\FakeLib\FSIHelper.fs:line 471
DeployScript.fsx(10,5): error FS0001: This expression was expected to have type

这里的重点是我正在尝试仅在某个目标下执行shell命令.不注意mkdir命令,因为我实际上想使用schtasks命令.我只是为了简单起见而使用了mkdir.也许我在这里缺少一个细微差别.我也注意到使用environVarOrFail时的这种行为.在此先感谢所有回复的人.

Answer 1

Shell.Exec返回一个int,但Target需要一个返回a的函数unit.

试试这个:

let returnUnit() = ()
let return42() = 42

Target "One" returnUnit // works
Target "Two" return42 // fails: expected to have type unit, but here has type int

目标"Two"无法编译,因为它的第二个参数是一个返回an int的函数,而一个函数返回unit是预期的.

这个错误可以让你忘记忘记函数返回值:编译器有用地告诉你"嘿,你确定你不想对这个int做任何事吗？那你为什么要首先调用这个函数？"

但是,有时候抛弃返回值是有意义的.例如,在这种特殊情况下,您要求Shell.Exec其副作用,并且其返回值不会引起您的兴趣.这种情况经常令人惊讶地发生,因此通常会有一个特殊的功能 - ignore.

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    Shell.Exec("mkdir","exampleDirectory") |> ignore // compiles fine
)

此函数接受任何值,抛出它并返回一个单位.通过将其应用于结果Shell.Exec,您告诉编译器:"是的,我确定我不需要这个值,请ignore它."

也就是说,我强烈建议使用专门的函数而不是通用的系统调用.这将使您的脚本更具可读性,甚至可以帮助您尽早发现错误.例如,要创建目录,请使用FileUtils.mkdir:

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    FileUtils.mkdir "exampleDirectory"  // no need for `ignore`, mkdir already returns `unit`
)