Update README to show how to call Rust code via GHCi (#12)

Author: kamoshi

README.md

@@ -245,6 +245,57 @@ Linking /Users/yvan/demo/dist-newstyle/build/aarch64-osx/ghc-9.0.2/test-0.1.0.0/
 Hello, Rust 🦀!
 ```
 
+Now let's see if we can use the GHCi repl to call the functions defined in Rust.
+
+```text
+λ> withCString "aaa" hello
+ghc-9.4.8: ^^ Could not load '__c_hello', dependency unresolved. See top entry above.
+
+
+GHC.ByteCode.Linker: can't find label
+During interactive linking, GHCi couldn't find the following symbol:
+  __c_hello
+This may be due to you not asking GHCi to load extra object files,
+archives or DLLs needed by your current session.  Restart GHCi, specifying
+the missing library using the -L/path/to/object/dir and -lmissinglibname
+flags, or simply by naming the relevant files on the GHCi command line.
+Alternatively, this link failure might indicate a bug in GHCi.
+If you suspect the latter, please report this as a GHC bug:
+  https://www.haskell.org/ghc/reportabug
+```
+
+It seems like GHCi doesn't know how to link the library containing external
+functions. To fix this we first need to change the type of the Rust crate to
+`cdylib` in `Cargo.toml`. The reason why we need to do this is that GHCi can
+only load dynamic libraries, not static ones.
+
+```toml
+[lib]
+crate-type = ["cdylib"]
+```
+Now we need to tell GHCi explicitly where to look for those libraries. We can
+do that by specifying the path in a `.ghci` file at the root of the project.
+
+```ghci
+:set -Ltarget/debug -lgreetings
+```
+
+After rebuilding the project with the necessary changes we can try once again:
+
+```text
+$ cabal repl
+Build profile: -w ghc-9.4.8 -O1
+In order, the following will be built (use -v for more details):
+ - greetings-0.1.0 (ephemeral targets)
+Preprocessing library for greetings-0.1.0..
+GHCi, version 9.4.8: https://www.haskell.org/ghc/  :? for help
+Loaded GHCi configuration from /path/to/project/greetings/.ghci
+[1 of 1] Compiling Greetings        ( src/Greetings.hs, interpreted )
+Ok, one module loaded.
+λ> withCString "Rust" hello
+Hello, Rust!
+```
+
 That's all folks! Happy hacking 🙂
 
 ## Nix support

src/main.rs

@@ -243,6 +243,57 @@
 //! Hello, Rust 🦀!
 //! ```
 //!
+//! Now let's see if we can use the GHCi repl to call the functions defined in Rust.
+//!
+//! ```text
+//! λ> withCString "aaa" hello
+//! ghc-9.4.8: ^^ Could not load '__c_hello', dependency unresolved. See top entry above.
+//!
+//!
+//! GHC.ByteCode.Linker: can't find label
+//! During interactive linking, GHCi couldn't find the following symbol:
+//!   __c_hello
+//! This may be due to you not asking GHCi to load extra object files,
+//! archives or DLLs needed by your current session.  Restart GHCi, specifying
+//! the missing library using the -L/path/to/object/dir and -lmissinglibname
+//! flags, or simply by naming the relevant files on the GHCi command line.
+//! Alternatively, this link failure might indicate a bug in GHCi.
+//! If you suspect the latter, please report this as a GHC bug:
+//!   https://www.haskell.org/ghc/reportabug
+//! ```
+//!
+//! It seems like GHCi doesn't know how to link the library containing external
+//! functions. To fix this we first need to change the type of the Rust crate to
+//! `cdylib` in `Cargo.toml`. The reason why we need to do this is that GHCi can
+//! only load dynamic libraries, not static ones.
+//!
+//! ```toml
+//! [lib]
+//! crate-type = ["cdylib"]
+//! ```
+//! Now we need to tell GHCi explicitly where to look for those libraries. We can
+//! do that by specifying the path in a `.ghci` file at the root of the project.
+//!
+//! ```ghci
+//! :set -Ltarget/debug -lgreetings
+//! ```
+//!
+//! After rebuilding the project with the necessary changes we can try once again:
+//!
+//! ```text
+//! $ cabal repl
+//! Build profile: -w ghc-9.4.8 -O1
+//! In order, the following will be built (use -v for more details):
+//!  - greetings-0.1.0 (ephemeral targets)
+//! Preprocessing library for greetings-0.1.0..
+//! GHCi, version 9.4.8: https://www.haskell.org/ghc/  :? for help
+//! Loaded GHCi configuration from /path/to/project/greetings/.ghci
+//! [1 of 1] Compiling Greetings        ( src/Greetings.hs, interpreted )
+//! Ok, one module loaded.
+//! λ> withCString "Rust" hello
+//! Hello, Rust!
+//! ```
+//!
 //! That's all folks! Happy hacking 🙂
 //!
 //! ## Nix support