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Oct 13, 2025 by wbrum.org 77 views

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Introduction

Typed Racket is a statically typed variant of the Racket programming language. It provides a type system that helps catch errors at compile-time, making it easier to write robust and maintainable code. In this article, we will explore the use of the in-directory function in Typed Racket.

What is in-directory?

The in-directory function is a part of the Racket standard library. It returns a sequence of paths that represent the files and directories in the current directory. The function takes two optional arguments: a boolean indicating whether to include the current directory itself, and a predicate function that filters the paths.

Using in-directory in Typed Racket

When using in-directory in Typed Racket, we need to specify the types of the arguments. The function signature is as follows:

- : (->* () ((U False Path-String) (-> Path Any)) (Sequenceof Path))

This means that the function takes two optional arguments: a boolean and a predicate function. The boolean can be either #f or a path string, and the predicate function takes a path as input and returns a value of any type.

Example Code

Here is an example of using in-directory in Typed Racket:

$ racket -l typed/racket -i
> in-directory
- : (->* () ((U False Path-String) (-> Path Any)) (Sequenceof Path))
#<procedure:in-directory>
> (for ((p (in-directory #f (lambda ((_ : Path-String)) #f))))
    p)
string:1:0: Type Checker: missing type for identifier;
 consider using `require/typed' to import it
  identifier: initial-state
  from module: for.rkt
  in: (for ((p (in-directory #f (lambda ((_ : Path-String)) #f)))) p)
 [,bt for context]

In this example, we are using the in-directory function to get a sequence of paths in the current directory. We are passing #f as the first argument, which means that we want to include the current directory itself. We are also passing a predicate function that takes a path as input and returns #f, which means that we want to filter out all the paths.

Error Message

However, when we run this code, we get the following error message:

string:1:0: Type Checker: missing type for identifier;
 consider using `require/typed' to import it
  identifier: initial-state
  from module: for.rkt
  in: (for ((p (in-directory #f (lambda ((_ : Path-String)) #f)))) p)
 [,bt for context]

This error message is telling us that the type checker is missing a type for the identifier initial-state. This is because we are using the in-directory function without specifying the types of the arguments.

Solution

To fix this error, we need to specify the types of the arguments. We can do this by using the require/typed form to import the in-directory function with the correct types. Here is an example of how we can do this:

#lang typed/racket

(require typed/racket
         (only-in racket/file in-directory))

(: in-directory-typed
   (->* () ((U False Path-String) (-> Path Any)) (Sequenceof Path)))

In this example, we are using the require/typed form to import the in-directory function with the correct types. We are specifying that the function takes two optional arguments: a boolean and a predicate function, and returns a sequence of paths.

Conclusion

In conclusion, using the in-directory function in Typed Racket requires specifying the types of the arguments. We can do this by using the require/typed form to import the function with the correct types. By following these steps, we can write robust and maintainable code in Typed Racket.

Additional Information

What version of Racket are you using?

$ racket -v
Welcome to Racket v8.16 [cs].

What program did you run?

$ racket -l typed/racket -i

What should have happened?

This form should have been evaluated successfully.

If you got an error message, please include it here.

string:1:0: Type Checker: missing type for identifier;
 consider using `require/typed' to import it
  identifier: initial-state
  from module: for.rkt
  in: (for ((p (in-directory #f (lambda ((_ : Path-String)) #f)))) p)
 [,bt for context]

Please include any other relevant details

$ uname -a
Linux fedora 6.13.5-102.bazzite.fc41.x86_64 #1 SMP PREEMPT_DYNAMIC Fri Feb 28 10:23:59 UTC 2025 x86_64 GNU/Linux
```<br/>
# **Use in-directory in Typed Racket: Q&A**
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## **Q: What is the purpose of the in-directory function in Typed Racket?**
----------------------------------------------------------------

A: The `in-directory` function in Typed Racket is used to return a sequence of paths that represent the files and directories in the current directory. It takes two optional arguments: a boolean indicating whether to include the current directory itself, and a predicate function that filters the paths.

## **Q: How do I use the in-directory function in Typed Racket?**
---------------------------------------------------------

A: To use the `in-directory` function in Typed Racket, you need to specify the types of the arguments. You can do this by using the `require/typed` form to import the function with the correct types.

## **Q: What is the correct syntax for using the in-directory function in Typed Racket?**
--------------------------------------------------------------------------------

A: The correct syntax for using the `in-directory` function in Typed Racket is as follows:
```racket
(in-directory [include-current-dir] [predicate])

Where include-current-dir is a boolean indicating whether to include the current directory itself, and predicate is a function that takes a path as input and returns a value of any type.

Q: How do I specify the types of the arguments for the in-directory function in Typed Racket?

A: To specify the types of the arguments for the in-directory function in Typed Racket, you can use the require/typed form to import the function with the correct types. For example:

#lang typed/racket

(require typed/racket
         (only-in racket/file in-directory))

(: in-directory-typed
   (->* () ((U False Path-String) (-> Path Any)) (Sequenceof Path)))

Q: What is the difference between the in-directory function and the directory-list function in Typed Racket?

A: The in-directory function and the directory-list function in Typed Racket are both used to return a list of paths in the current directory. However, the in-directory function takes two optional arguments: a boolean indicating whether to include the current directory itself, and a predicate function that filters the paths. The directory-list function, on the other hand, only returns a list of paths in the current directory.

Q: How do I filter the paths returned by the in-directory function in Typed Racket?

A: To filter the paths returned by the in-directory function in Typed Racket, you can pass a predicate function as the second argument to the function. The predicate function takes a path as input and returns a value of any type. For example:

(in-directory #f (lambda (path) (string? path)))

This will return a list of paths in the current directory that are strings.

Q: What is the type of the paths returned by the in-directory function in Typed Racket?

A: The type of the paths returned by the in-directory function in Typed Racket is Path. This is a type that represents a path in the file system.

Q: How do I get the current directory in Typed Racket?

A: To get the current directory in Typed Racket, you can use the current-directory function. This function returns the current working directory as a Path.

Q: What is the difference between the current-directory function and the in-directory function in Typed Racket?

A: The current-directory function and the in-directory function in Typed Racket are both used to get the current directory. However, the current-directory function returns the current working directory as a Path, while the in-directory function returns a list of paths in the current directory.

Q: How do I get the list of files in the current directory in Typed Racket?

A: To get the list of files in the current directory in Typed Racket, you can use the in-directory function with a predicate function that filters out directories. For example:

(in-directory #f (lambda (path) (file? path)))

This will return a list of files in the current directory.

Q: What is the type of the list of files returned by the in-directory function in Typed Racket?

A: The type of the list of files returned by the in-directory function in Typed Racket is (Sequenceof Path). This is a type that represents a sequence of paths.

Q: How do I get the list of directories in the current directory in Typed Racket?

A: To get the list of directories in the current directory in Typed Racket, you can use the in-directory function with a predicate function that filters out files. For example:

(in-directory #f (lambda (path) (directory? path)))

This will return a list of directories in the current directory.

Q: What is the type of the list of directories returned by the in-directory function in Typed Racket?

A: The type of the list of directories returned by the in-directory function in Typed Racket is (Sequenceof Path). This is a type that represents a sequence of paths.

Q: How do I get the list of files and directories in the current directory in Typed Racket?

A: To get the list of files and directories in the current directory in Typed Racket, you can use the in-directory function without any predicate function. For example:

(in-directory #f (lambda (path) #t))

This will return a list of files and directories in the current directory.

Q: What is the type of the list of files and directories returned by the in-directory function in Typed Racket?

A: The type of the list of files and directories returned by the in-directory function in Typed Racket is (Sequenceof Path). This is a type that represents a sequence of paths.