How to Use the GHC (Glasgow Haskell Compiler) Options?

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The Glasgow Haskell Compiler (GHC) is a widely used compiler for the Haskell programming language. It provides several command-line options to control various aspects of the compilation process and behavior. Here are some common GHC options and their usage:

  • -Wall: Enables enabling all optional warnings. It helps catch potential issues and promotes better coding practices.
  • -O and -O2: Enable optimization levels 1 and 2, respectively. They improve the performance of the compiled code by applying various optimizations.
  • -o : Specifies the name of the output file generated by the compiler.
  • -c: Compiles but does not link the code, generating object files instead.
  • -i: Adds a search directory for modules. The compiler looks for imported modules in these directories.
  • -main-is : Specifies the entry point module of the program.
  • -hidir and -odir : Specify the directory for storing generated interface (-hidir) and object (-odir) files.
  • -l and -L: Links against a specific library (-l) and searches for libraries in a given directory (-L).
  • -threaded: Enables support for threaded runtime, allowing concurrent execution.
  • -cpp: Enables preprocessing using the C preprocessor. Useful for conditional compilation or code generation.
  • -X: Enables a specific language extension. For example, -XDataKinds enables the DataKinds extension.

These are just a few examples of the many options provided by GHC. To get a comprehensive list of options with their descriptions and usage, you can run ghc --help on the command line or refer to the GHC User's Guide available online.

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What is the -X flag used for in GHC options?

The -X flag is used in GHC (Glasgow Haskell Compiler) options to enable language extensions. GHC provides a number of language extensions that add new features or change the behavior of the Haskell language.

By using the -X flag followed by a specific extension, you can enable that extension in your Haskell source code. For example, if you want to enable the ScopedTypeVariables extension, you would use the -XScopedTypeVariables flag.

Here's an example of how you can compile a Haskell program with the -X flag:

ghc -XScopedTypeVariables myprogram.hs

This command enables the ScopedTypeVariables extension while compiling the "myprogram.hs" file. The extension allows you to explicitly specify type variables in the scope of a function, even if they are not used in the function's type signature.

Note that you can also enable multiple language extensions by providing multiple -X flags:

ghc -XScopedTypeVariables -XTypeApplications myprogram.hs

In this case, both the ScopedTypeVariables and TypeApplications extensions are enabled.

How to specify the target language version in GHC options?

To specify the target language version in GHC (Glasgow Haskell Compiler) options, you can use the -X flag followed by the desired language version.

Here's an example of how you can specify the target language version using GHC options:

  1. Open a terminal or command prompt.
  2. Navigate to the directory containing your Haskell source file (.hs file).
  3. Run the ghc command followed by the GHC options. For example, if you want to specify Haskell 2010 as the target language version, use the -XHaskell2010 option: ghc -XHaskell2010 MyProgram.hs If you're using a different language version, replace Haskell2010 with the desired version identifier.
  4. GHC will compile your Haskell source file using the specified language version. The generated executable file will be named MyProgram.

Note: The available language versions vary depending on the GHC version you're using. You can check the GHC documentation or use the ghc --supported-languages command to see the list of supported language versions.

How to enforce safe Haskell code using GHC options?

To enforce safe Haskell code using GHC options, you can use the "-XSafe" option followed by specific safe language extensions. Here's how you can do it:

  1. Open your Haskell source file in a text editor or IDE.
  2. Add the following line at the top of your file to enable safe Haskell:
{-# LANGUAGE Safe #-}

This language pragma tells GHC that you want to enable the safe language extension.

  1. Use specific safe language extensions to enforce additional safety measures in your code. Some commonly used safe language extensions include:
  • -XSafeImports: Restricts imports to safe modules only.
  • -XTrustworthy: Allows explicit marking of trusted modules. (Use with caution and avoid using unless necessary)

You can add these options to your GHC command line or build system configuration file (e.g., stack.yaml or cabal.project).

If you're using GHC directly from the command line, you can pass the options as follows:

ghc -XSafe -XSafeImports MySafeCode.hs

Make sure to replace "MySafeCode.hs" with the actual name of your Haskell source file.

Enforcing safe Haskell code using GHC options helps you ensure that your code adheres to strict safety rules and guards against potentially unsafe operations.

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