diff --git a/milli/src/error.rs b/milli/src/error.rs
index c6dc85e9c..ce22a5512 100644
--- a/milli/src/error.rs
+++ b/milli/src/error.rs
@@ -1,60 +1,50 @@
-use std::io;
+use std::error::Error as StdError;
+use std::{fmt, io};
+
+use heed::{MdbError, Error as HeedError};
+use serde_json::{Map, Value};
 
 use crate::{DocumentId, FieldId};
-use heed::{MdbError, Error as HeedError};
 
-pub type Object = serde_json::Map<String, serde_json::Value>;
+pub type Object = Map<String, Value>;
 
+#[derive(Debug)]
 pub enum Error {
     InternalError(InternalError),
     IoError(io::Error),
     UserError(UserError),
 }
 
+#[derive(Debug)]
 pub enum InternalError {
-    DatabaseMissingEntry(DatabaseMissingEntry),
+    DatabaseMissingEntry { db_name: &'static str, key: Option<&'static str> },
     FieldIdMapMissingEntry(FieldIdMapMissingEntry),
-    IndexingMergingKeys(IndexingMergingKeys),
+    IndexingMergingKeys { process: &'static str },
     SerializationError(SerializationError),
     StoreError(MdbError),
     InvalidDatabaseTyping,
     DatabaseClosing,
 }
 
+#[derive(Debug)]
 pub enum SerializationError {
     Decoding { db_name: Option<&'static str> },
     Encoding { db_name: Option<&'static str> },
     InvalidNumberSerialization,
 }
 
-pub enum IndexingMergingKeys {
-    DocIdWordPosition,
-    Document,
-    MainFstDeserialization,
-    WordLevelPositionDocids,
-    WordPrefixLevelPositionDocids,
-}
-
+#[derive(Debug)]
 pub enum FieldIdMapMissingEntry {
-    DisplayedFieldId { field_id: FieldId },
-    DisplayedFieldName { field_name: String },
-    FacetedFieldName { field_name: String },
-    FilterableFieldName { field_name: String },
-    SearchableFieldName { field_name: String },
-}
-
-pub enum DatabaseMissingEntry {
-    DocumentId { internal_id: DocumentId },
-    FacetValuesDocids,
-    IndexCreationTime,
-    IndexUpdateTime,
+    FieldId { field_id: FieldId, from_db_name: &'static str },
+    FieldName { field_name: String, from_db_name: &'static str },
 }
 
+#[derive(Debug)]
 pub enum UserError {
     AttributeLimitReached,
     DocumentLimitReached,
     InvalidCriterionName { name: String },
-    InvalidDocumentId { document_id: DocumentId },
+    InvalidDocumentId { document_id: Value },
     MissingDocumentId { document: Object },
     MissingPrimaryKey,
     DatabaseSizeReached,
@@ -88,3 +78,93 @@ impl From<HeedError> for Error {
         }
     }
 }
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::InternalError(error) => write!(f, "internal: {}", error),
+            Self::IoError(error) => error.fmt(f),
+            Self::UserError(error) => error.fmt(f),
+        }
+    }
+}
+
+impl StdError for Error {}
+
+impl fmt::Display for InternalError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::DatabaseMissingEntry { db_name, key } => {
+                write!(f, "missing {} in the {} database", key.unwrap_or("key"), db_name)
+            },
+            Self::FieldIdMapMissingEntry(error) => error.fmt(f),
+            Self::IndexingMergingKeys { process } => {
+                write!(f, "invalid merge while processing {}", process)
+            },
+            Self::SerializationError(error) => error.fmt(f),
+            Self::StoreError(error) => error.fmt(f),
+            Self::InvalidDatabaseTyping => HeedError::InvalidDatabaseTyping.fmt(f),
+            Self::DatabaseClosing => HeedError::DatabaseClosing.fmt(f),
+        }
+    }
+}
+
+impl StdError for InternalError {}
+
+impl fmt::Display for UserError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::AttributeLimitReached => f.write_str("maximum number of attributes reached"),
+            Self::DocumentLimitReached => f.write_str("maximum number of documents reached"),
+            Self::InvalidCriterionName { name } => write!(f, "invalid criterion {}", name),
+            Self::InvalidDocumentId { document_id } => {
+                let json = serde_json::to_string(document_id).unwrap();
+                write!(f, "document identifier is invalid {}", json)
+            },
+            Self::MissingDocumentId { document } => {
+                let json = serde_json::to_string(document).unwrap();
+                write!(f, "document doesn't have an identifier {}", json)
+            },
+            Self::MissingPrimaryKey => f.write_str("missing primary key"),
+            Self::DatabaseSizeReached => f.write_str("database size reached"),
+            // TODO where can we find it instead of writing the text ourselves?
+            Self::NoSpaceLeftOnDevice => f.write_str("no space left on device"),
+            Self::InvalidStoreFile => f.write_str("store file is not a valid database file"),
+        }
+    }
+}
+
+impl StdError for UserError {}
+
+impl fmt::Display for FieldIdMapMissingEntry {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::FieldId { field_id, from_db_name } => {
+                write!(f, "unknown field id {} coming from {} database", field_id, from_db_name)
+            },
+            Self::FieldName { field_name, from_db_name } => {
+                write!(f, "unknown field name {} coming from {} database", field_name, from_db_name)
+            },
+        }
+    }
+}
+
+impl StdError for FieldIdMapMissingEntry {}
+
+impl fmt::Display for SerializationError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Decoding { db_name: Some(name) } => {
+                write!(f, "decoding from the {} database failed", name)
+            },
+            Self::Decoding { db_name: None } => f.write_str("decoding failed"),
+            Self::Encoding { db_name: Some(name) } => {
+                write!(f, "encoding into the {} database failed", name)
+            },
+            Self::Encoding { db_name: None } => f.write_str("encoding failed"),
+            Self::InvalidNumberSerialization => f.write_str("number is not a valid finite number"),
+        }
+    }
+}
+
+impl StdError for SerializationError {}