Merge pull request #165 from fteychene/feat/resolve_cache

3 files changed, 707 insertions, 8 deletions

diff --git a/dhall/src/semantics/resolve/cache.rs b/dhall/src/semantics/resolve/cache.rs
new file mode 100644
index 0000000..b725bf3
--- /dev/null
+++ b/dhall/src/semantics/resolve/cache.rs
@@ -0,0 +1,687 @@
+use std::env;
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+use crate::error::{CacheError, Error, ErrorKind};
+use crate::parse::parse_binary_file;
+use crate::semantics::{Import, TypedHir};
+use crate::syntax::Hash;
+use crate::syntax::{binary, Expr};
+use crate::Parsed;
+use std::env::VarError;
+use std::ffi::OsStr;
+use std::fs::File;
+
+#[cfg(unix)]
+const ALTERNATE_ENV_VAR: &str = "HOME";
+
+#[cfg(windows)]
+const ALTERNATE_ENV_VAR: &str = "LOCALAPPDATA";
+
+fn alternate_env_var_cache_dir(
+    provider: impl Fn(&str) -> Result<String, VarError>,
+) -> Option<PathBuf> {
+    provider(ALTERNATE_ENV_VAR)
+        .map(PathBuf::from)
+        .map(|env_dir| env_dir.join(".cache").join("dhall"))
+        .ok()
+}
+
+fn env_var_cache_dir(
+    provider: impl Fn(&str) -> Result<String, VarError>,
+) -> Option<PathBuf> {
+    provider("XDG_CACHE_HOME")
+        .map(PathBuf::from)
+        .map(|cache_home| cache_home.join("dhall"))
+        .ok()
+}
+
+fn load_cache_dir(
+    provider: impl Fn(&str) -> Result<String, VarError> + Copy,
+) -> Result<PathBuf, CacheError> {
+    env_var_cache_dir(provider)
+        .or_else(|| alternate_env_var_cache_dir(provider))
+        .ok_or(CacheError::MissingConfiguration)
+}
+
+#[derive(Debug, PartialEq)]
+pub struct Cache {
+    cache_dir: Option<PathBuf>,
+}
+
+impl Cache {
+    fn new_with_provider(
+        provider: impl Fn(&str) -> Result<String, VarError> + Copy,
+    ) -> Cache {
+        // Should warn that we can't initialize cache on error
+        let cache_dir = load_cache_dir(provider).and_then(|path| {
+            if !path.exists() {
+                std::fs::create_dir_all(path.as_path())
+                    .map(|_| path)
+                    .map_err(|e| CacheError::InitialisationError { cause: e })
+            } else {
+                Ok(path)
+            }
+        });
+        Cache {
+            cache_dir: cache_dir.ok(),
+        }
+    }
+
+    pub fn new() -> Cache {
+        Cache::new_with_provider(|name| env::var(OsStr::new(name)))
+    }
+}
+
+impl Cache {
+    fn cache_file(&self, import: &Import) -> Option<PathBuf> {
+        self.cache_dir
+            .as_ref()
+            .and_then(|cache_dir| {
+                import.hash.as_ref().map(|hash| (cache_dir, hash))
+            })
+            .map(|(cache_dir, hash)| cache_dir.join(cache_filename(hash)))
+    }
+
+    fn search_cache_file(&self, import: &Import) -> Option<PathBuf> {
+        self.cache_file(import)
+            .filter(|cache_file| cache_file.exists())
+    }
+
+    fn search_cache(&self, import: &Import) -> Option<Result<Parsed, Error>> {
+        self.search_cache_file(import)
+            .map(|cache_file| parse_binary_file(cache_file.as_path()))
+    }
+
+    // Side effect since we don't use the result
+    fn delete_cache(&self, import: &Import) {
+        self.search_cache_file(import)
+            .map(|cache_file| std::fs::remove_file(cache_file.as_path()));
+    }
+
+    // Side effect since we don't use the result
+    fn save_expr(&self, import: &Import, expr: &Expr) {
+        self.cache_file(import)
+            .map(|cache_file| save_expr(cache_file.as_path(), expr));
+    }
+
+    pub fn caching_import<F, R>(
+        &self,
+        import: &Import,
+        fetcher: F,
+        mut resolver: R,
+    ) -> Result<TypedHir, Error>
+    where
+        F: FnOnce() -> Result<Parsed, Error>,
+        R: FnMut(Parsed) -> Result<TypedHir, Error>,
+    {
+        // Lookup the cache
+        self.search_cache(import)
+            // On cache found
+            .and_then(|cache_result| {
+                // Try to resolve the cache imported content
+                match cache_result.and_then(|parsed| resolver(parsed)).and_then(
+                    |typed_hir| {
+                        check_hash(import.hash.as_ref().unwrap(), typed_hir)
+                    },
+                ) {
+                    // Cache content is invalid (can't be parsed / can't be resolved / content sha invalid )
+                    Err(_) => {
+                        // Delete cache file since it's invalid
+                        self.delete_cache(import);
+                        // Result as there were no cache
+                        None
+                    }
+                    // Cache valid
+                    r => Some(r),
+                }
+            })
+            .unwrap_or_else(|| {
+                // Fetch and resolve as provided
+                let imported = fetcher().and_then(resolver);
+                // Save in cache the result if ok
+                let _ = imported.as_ref().map(|(hir, _)| {
+                    self.save_expr(import, &hir.to_expr_noopts())
+                });
+                imported
+            })
+    }
+}
+
+fn save_expr(file_path: &Path, expr: &Expr) -> Result<(), Error> {
+    File::create(file_path)?.write_all(binary::encode(expr)?.as_slice())?;
+    Ok(())
+}
+
+fn check_hash(hash: &Hash, typed_hir: TypedHir) -> Result<TypedHir, Error> {
+    if hash.as_ref()[..] != typed_hir.0.to_expr_alpha().hash()?[..] {
+        Err(Error::new(ErrorKind::Cache(CacheError::CacheHashInvalid)))
+    } else {
+        Ok(typed_hir)
+    }
+}
+
+fn cache_filename<A: AsRef<[u8]>>(v: A) -> String {
+    format!("1220{}", hex::encode(v.as_ref()))
+}
+
+impl AsRef<[u8]> for Hash {
+    fn as_ref(&self) -> &[u8] {
+        match self {
+            Hash::SHA256(sha) => sha.as_slice(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::semantics::parse::parse_str;
+    use crate::syntax::{
+        parse_expr, ExprKind, ImportMode, ImportTarget, NumKind, Span,
+    };
+    use rand::distributions::Alphanumeric;
+    use rand::Rng;
+    use std::env::temp_dir;
+
+    #[cfg(unix)]
+    #[test]
+    fn alternate_env_var_cache_dir_should_result_unix_folder_path() {
+        let actual =
+            alternate_env_var_cache_dir(|_| Ok("/home/user".to_string()));
+        assert_eq!(actual, Some(PathBuf::from("/home/user/.cache/dhall")));
+    }
+
+    #[test]
+    fn alternate_env_var_cache_dir_should_result_none_on_missing_var() {
+        let actual = alternate_env_var_cache_dir(|_| Err(VarError::NotPresent));
+        assert_eq!(actual, None);
+    }
+
+    #[test]
+    fn env_var_cache_dir_should_result_xdg_cache_home() {
+        let actual = env_var_cache_dir(|_| {
+            Ok("/home/user/custom/path/for/cache".to_string())
+        });
+        assert_eq!(
+            actual,
+            Some(PathBuf::from("/home/user/custom/path/for/cache/dhall"))
+        );
+    }
+
+    #[test]
+    fn env_var_cache_dir_should_result_none_on_missing_var() {
+        let actual = env_var_cache_dir(|_| Err(VarError::NotPresent));
+        assert_eq!(actual, None);
+    }
+
+    #[test]
+    fn load_cache_dir_should_result_xdg_cache_first() {
+        let actual = load_cache_dir(|var| match var {
+            "XDG_CACHE_HOME" => Ok("/home/user/custom".to_string()),
+            _ => Err(VarError::NotPresent),
+        });
+        assert_eq!(actual.unwrap(), PathBuf::from("/home/user/custom/dhall"));
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn load_cache_dir_should_result_alternate() {
+        let actual = load_cache_dir(|var| match var {
+            ALTERNATE_ENV_VAR => Ok("/home/user".to_string()),
+            _ => Err(VarError::NotPresent),
+        });
+        assert_eq!(actual.unwrap(), PathBuf::from("/home/user/.cache/dhall"));
+    }
+
+    #[test]
+    fn load_cache_dir_should_result_none() {
+        let actual = load_cache_dir(|_| Err(VarError::NotPresent));
+        assert!(matches!(
+            actual.unwrap_err(),
+            CacheError::MissingConfiguration
+        ));
+    }
+
+    #[test]
+    fn new_with_provider_should_create_cache_folder() {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path()).unwrap();
+
+        let actual = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+        assert_eq!(
+            actual,
+            Cache {
+                cache_dir: Some(dir.join("dhall"))
+            }
+        );
+        assert!(dir.join("dhall").exists());
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+    }
+
+    #[test]
+    fn new_with_provider_should_return_cache_for_existing_folder() {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path()).unwrap();
+        File::create(dir.join("dhall")).unwrap();
+
+        assert!(dir.join("dhall").exists());
+
+        let actual = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+        assert_eq!(
+            actual,
+            Cache {
+                cache_dir: Some(dir.join("dhall"))
+            }
+        );
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+    }
+
+    #[test]
+    fn caching_import_should_load_cache() -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+
+        // Create cache file
+        let expr =
+            Expr::new(ExprKind::Num(NumKind::Natural(1)), Span::Artificial);
+        File::create(dir.join("dhall").join("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15"))?
+            .write_all(binary::encode(&expr)?.as_ref())?;
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || panic!("Should not fetch import"),
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok());
+        assert_eq!(resolve_counter, 1);
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_skip_cache_if_missing_cache_folder(
+    ) -> Result<(), Error> {
+        let cache = Cache::new_with_provider(|_| Err(VarError::NotPresent));
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut resolve_counter = 0;
+        let mut fetcher_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(resolve_counter, 1);
+        assert_eq!(fetcher_counter, 1);
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_skip_cache_on_no_hash_import() -> Result<(), Error>
+    {
+        let cache = Cache::new_with_provider(|_| Err(VarError::NotPresent));
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: None,
+        };
+
+        let mut resolve_counter = 0;
+        let mut fetcher_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(resolve_counter, 1);
+        assert_eq!(fetcher_counter, 1);
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_fetch_import_if_no_cache() -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut fetcher_counter = 0;
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(resolve_counter, 1);
+        assert_eq!(fetcher_counter, 1);
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_fetch_import_on_cache_parsed_error(
+    ) -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+
+        File::create(dir.join("dhall").join("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15"))?
+            .write_all("Invalid content".as_bytes())?;
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut fetcher_counter = 0;
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(fetcher_counter, 1, "Should fetch since cache is invalid");
+        assert_eq!(
+            resolve_counter, 1,
+            "Should resolve only 1 time because cache can't be parsed"
+        );
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_fetch_import_on_cache_resolve_error(
+    ) -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+
+        let expr =
+            Expr::new(ExprKind::Num(NumKind::Natural(2)), Span::Artificial);
+        File::create(dir.join("dhall").join("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15"))?
+            .write_all(binary::encode(&expr)?.as_slice())?;
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut fetcher_counter = 0;
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                match resolve_counter {
+                    1 => Err(Error::new(ErrorKind::Cache(
+                        CacheError::CacheHashInvalid,
+                    ))),
+                    _ => {
+                        let result = parsed.resolve()?.typecheck()?;
+                        Ok((result.normalize().to_hir(), result.ty))
+                    }
+                }
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(fetcher_counter, 1, "Should fetch since cache is invalid");
+        assert_eq!(
+            resolve_counter, 2,
+            "Should resolve 2 time (one for cache that fail, one for fetch)"
+        );
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_fetch_import_on_invalid_hash_cache_content(
+    ) -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+
+        let expr =
+            Expr::new(ExprKind::Num(NumKind::Natural(2)), Span::Artificial);
+        File::create(dir.join("dhall").join("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15"))?
+            .write_all(binary::encode(&expr)?.as_slice())?;
+
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut fetcher_counter = 0;
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(fetcher_counter, 1, "Should fetch since cache is invalid");
+        assert_eq!(
+            resolve_counter, 2,
+            "Should resolve 2 time (one for cache, one for fetch)"
+        );
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn caching_import_should_save_import_if_missing() -> Result<(), Error> {
+        let test_id = rand::thread_rng()
+            .sample_iter(Alphanumeric)
+            .take(36)
+            .collect::<String>();
+        let dir = temp_dir().join(test_id);
+
+        std::fs::create_dir_all(dir.as_path())?;
+
+        let cache = Cache::new_with_provider(|_| {
+            Ok(dir.clone().to_str().map(String::from).unwrap())
+        });
+        let import = Import {
+            mode: ImportMode::Code,
+            location: ImportTarget::Missing,
+            hash: Some(Hash::SHA256(hex::decode("d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15").unwrap())),
+        };
+
+        let mut fetcher_counter = 0;
+        let mut resolve_counter = 0;
+
+        let result = cache.caching_import(
+            &import,
+            || {
+                fetcher_counter += 1;
+                parse_str("1")
+            },
+            |parsed| {
+                resolve_counter += 1;
+                let result = parsed.resolve()?.typecheck()?;
+                Ok((result.normalize().to_hir(), result.ty))
+            },
+        );
+
+        assert!(result.is_ok(), "caching_import Should be valid");
+        assert_eq!(fetcher_counter, 1, "Should fetch since cache is mising");
+        assert_eq!(resolve_counter, 1, "Should resolve 1 time");
+
+        let cache_file = dir.join("dhall").join("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15");
+        assert!(cache_file.exists());
+
+        std::fs::remove_dir_all(dir.as_path()).unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn cache_filename_should_result_for_hash() {
+        let hash =
+            Hash::SHA256(parse_expr("1").unwrap().hash().unwrap().into_vec());
+        assert_eq!("1220d60d8415e36e86dae7f42933d3b0c4fe3ca238f057fba206c7e9fbf5d784fe15".to_string(), cache_filename(hash));
+    }
+
+    #[test]
+    fn check_hash_should_be_ok_for_same_hash() -> Result<(), Error> {
+        let typed = parse_str("1")?.resolve()?.typecheck()?;
+        let hash = Hash::SHA256(parse_expr("1")?.hash()?.into_vec());
+
+        let expected = (typed.normalize().to_hir(), typed.ty);
+        let actual = check_hash(&hash, expected.clone());
+        assert_eq!(actual.unwrap(), expected);
+        Ok(())
+    }
+
+    #[test]
+    fn check_hash_should_be_ok_for_unmatching_hash() -> Result<(), Error> {
+        let typed = parse_str("1")?.resolve()?.typecheck()?;
+        let hash = Hash::SHA256(parse_expr("2")?.hash()?.into_vec());
+
+        let expected = (typed.normalize().to_hir(), typed.ty);
+        let actual = check_hash(&hash, expected);
+        assert!(actual.is_err());
+        Ok(())
+    }
+}
diff --git a/dhall/src/semantics/resolve/mod.rs b/dhall/src/semantics/resolve/mod.rs
index 33b477e..b79cf65 100644
--- a/dhall/src/semantics/resolve/mod.rs
+++ b/dhall/src/semantics/resolve/mod.rs
@@ -1,6 +1,8 @@
+pub mod cache;
 pub mod env;
 pub mod hir;
 pub mod resolve;
+pub use cache::*;
 pub use env::*;
 pub use hir::*;
 pub use resolve::*;
diff --git a/dhall/src/semantics/resolve/resolve.rs b/dhall/src/semantics/resolve/resolve.rs
index e96f16b..3b72b9e 100644
--- a/dhall/src/semantics/resolve/resolve.rs
+++ b/dhall/src/semantics/resolve/resolve.rs
@@ -9,7 +9,7 @@ use crate::builtins::Builtin;
 use crate::error::ErrorBuilder;
 use crate::error::{Error, ImportError};
 use crate::operations::{BinOp, OpKind};
-use crate::semantics::{mkerr, Hir, HirKind, ImportEnv, NameEnv, Type};
+use crate::semantics::{mkerr, Cache, Hir, HirKind, ImportEnv, NameEnv, Type};
 use crate::syntax;
 use crate::syntax::{
     Expr, ExprKind, FilePath, FilePrefix, Hash, ImportMode, ImportTarget, Span,
@@ -209,6 +209,7 @@ fn make_aslocation_uniontype() -> Expr {
 
 fn resolve_one_import(
     env: &mut ImportEnv,
+    cache: &Cache,
     import: &Import,
     location: &ImportLocation,
     span: Span,
@@ -217,10 +218,16 @@ fn resolve_one_import(
     let location = location.chain(&import.location, do_sanity_check)?;
     env.handle_import(location.clone(), |env| match import.mode {
         ImportMode::Code => {
-            let parsed = location.fetch_dhall()?;
-            let typed = resolve_with_env(env, parsed)?.typecheck()?;
-            let hir = typed.normalize().to_hir();
-            let ty = typed.ty().clone();
+            let (hir, ty) = cache.caching_import(
+                import,
+                || location.fetch_dhall(),
+                |parsed| {
+                    let typed =
+                        resolve_with_env(env, cache, parsed)?.typecheck()?;
+                    let hir = typed.normalize().to_hir();
+                    Ok((hir, typed.ty))
+                },
+            )?;
             match &import.hash {
                 Some(Hash::SHA256(hash)) => {
                     let actual_hash = hir.to_expr_alpha().hash()?;
@@ -346,19 +353,23 @@ fn traverse_resolve_expr(
 
 fn resolve_with_env(
     env: &mut ImportEnv,
+    cache: &Cache,
     parsed: Parsed,
 ) -> Result<Resolved, Error> {
     let Parsed(expr, location) = parsed;
     let resolved = traverse_resolve_expr(
         &mut NameEnv::new(),
         &expr,
-        &mut |import, span| resolve_one_import(env, &import, &location, span),
+        &mut |import, span| {
+            resolve_one_import(env, cache, &import, &location, span)
+        },
     )?;
     Ok(Resolved(resolved))
 }
 
 pub fn resolve(parsed: Parsed) -> Result<Resolved, Error> {
-    resolve_with_env(&mut ImportEnv::new(), parsed)
+    let cache = Cache::new();
+    resolve_with_env(&mut ImportEnv::new(), &cache, parsed)
 }
 
 pub fn skip_resolve_expr(expr: &Expr) -> Result<Hir, Error> {
@@ -387,7 +398,6 @@ impl Canonicalize for FilePath {
                 // ───────────────────────────────────────
                 // canonicalize(directory₀/.) = directory₁
                 "." => continue,
-
                 ".." => match file_path.last() {
                     // canonicalize(directory₀) = ε
                     // ────────────────────────────