diff --git a/.gitignore b/.gitignore
index 9026c77a5..9ab2ce727 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /target
 .vscode
+src/tests/snapshots/snapshots
diff --git a/Cargo.lock b/Cargo.lock
index cfec233e5..06e0b8908 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -262,7 +262,7 @@ checksum = "721cae7de5c34fbb2acd27e21e6d2cf7b886dce0c27388d46c4e6c47ea4318dd"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -344,7 +344,7 @@ checksum = "57d123550fa8d071b7255cb0cc04dc302baa6c8c4a79f55701552684d8399bce"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -390,7 +390,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "85b6598a2f5d564fb7855dc6b06fd1c38cff5a72bd8b863a4d021938497b440a"
 dependencies = [
  "serde",
- "thiserror",
+ "thiserror 1.0.61",
 ]
 
 [[package]]
@@ -399,6 +399,12 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3a8241f3ebb85c056b509d4327ad0358fbbba6ffb340bf388f26350aeda225b1"
 
+[[package]]
+name = "bitflags"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
+
 [[package]]
 name = "bitflags"
 version = "2.5.0"
@@ -444,6 +450,16 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "bstr"
+version = "1.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1a68f1f47cdf0ec8ee4b941b2eee2a80cb796db73118c0dd09ac63fbe405be22"
+dependencies = [
+ "memchr",
+ "serde",
+]
+
 [[package]]
 name = "bumpalo"
 version = "3.16.0"
@@ -514,7 +530,7 @@ dependencies = [
  "rug",
  "serde",
  "serde_json",
- "thiserror",
+ "thiserror 1.0.61",
 ]
 
 [[package]]
@@ -601,7 +617,7 @@ dependencies = [
  "heck 0.5.0",
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -616,6 +632,20 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b6a852b24ab71dffc585bcb46eaf7959d175cb865a7152e35b348d1b2960422"
 
+[[package]]
+name = "console"
+version = "0.11.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8c0994e656bba7b922d8dd1245db90672ffb701e684e45be58f20719d69abc5a"
+dependencies = [
+ "encode_unicode",
+ "lazy_static",
+ "libc",
+ "terminal_size",
+ "termios",
+ "winapi",
+]
+
 [[package]]
 name = "constant_tracking"
 version = "2.0.0"
@@ -725,7 +755,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "strsim 0.11.1",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -747,7 +777,7 @@ checksum = "733cabb43482b1a1b53eee8583c2b9e8684d592215ea83efd305dd31bc2f0178"
 dependencies = [
  "darling_core 0.20.9",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -792,6 +822,12 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "difference"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "524cbf6897b527295dff137cec09ecf3a05f4fddffd7dfcd1585403449e74198"
+
 [[package]]
 name = "digest"
 version = "0.9.0"
@@ -847,7 +883,7 @@ dependencies = [
  "enum-ordinalize",
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -865,6 +901,12 @@ dependencies = [
  "log",
 ]
 
+[[package]]
+name = "encode_unicode"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a357d28ed41a50f9c765dbfe56cbc04a64e53e5fc58ba79fbc34c10ef3df831f"
+
 [[package]]
 name = "enum-ordinalize"
 version = "4.3.0"
@@ -882,7 +924,7 @@ checksum = "0d28318a75d4aead5c4db25382e8ef717932d0346600cacae6357eb5941bc5ff"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1021,7 +1063,7 @@ checksum = "87750cf4b7a4c0625b1529e4c543c2182106e4dedc60a2a6455e00d212c489ac"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1113,6 +1155,30 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d2fabcfbdc87f4758337ca535fb41a6d701b65693ce38287d856d1674551ec9b"
 
+[[package]]
+name = "globset"
+version = "0.4.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "15f1ce686646e7f1e19bf7d5533fe443a45dbfb990e00629110797578b42fb19"
+dependencies = [
+ "aho-corasick",
+ "bstr",
+ "log",
+ "regex-automata",
+ "regex-syntax 0.8.4",
+]
+
+[[package]]
+name = "globwalk"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93e3af942408868f6934a7b85134a3230832b9977cf66125df2f9edcfce4ddcc"
+dependencies = [
+ "bitflags 1.3.2",
+ "ignore",
+ "walkdir",
+]
+
 [[package]]
 name = "gmp-mpfr-sys"
 version = "1.6.4"
@@ -1308,6 +1374,22 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b9e0384b61958566e926dc50660321d12159025e767c18e043daf26b70104c39"
 
+[[package]]
+name = "ignore"
+version = "0.4.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6d89fd380afde86567dfba715db065673989d6253f42b88179abd3eae47bda4b"
+dependencies = [
+ "crossbeam-deque",
+ "globset",
+ "log",
+ "memchr",
+ "regex-automata",
+ "same-file",
+ "walkdir",
+ "winapi-util",
+]
+
 [[package]]
 name = "im"
 version = "15.1.0"
@@ -1344,6 +1426,23 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "insta"
+version = "0.16.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "617e921abc813f96a3b00958c079e7bf1e2db998f8a04f1546dd967373a418ee"
+dependencies = [
+ "console",
+ "difference",
+ "globwalk",
+ "lazy_static",
+ "pest",
+ "pest_derive",
+ "serde",
+ "serde_json",
+ "serde_yaml",
+]
+
 [[package]]
 name = "internal-tracing"
 version = "0.1.0"
@@ -1434,7 +1533,7 @@ dependencies = [
  "serde_with 1.14.0",
  "strum",
  "strum_macros",
- "thiserror",
+ "thiserror 1.0.61",
  "turshi",
 ]
 
@@ -1465,10 +1564,16 @@ version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c0ff37bd590ca25063e35af745c343cb7a0271906fb7b37e4813e8f79f00268d"
 dependencies = [
- "bitflags",
+ "bitflags 2.5.0",
  "libc",
 ]
 
+[[package]]
+name = "linked-hash-map"
+version = "0.5.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0717cef1bc8b636c6e1c1bbdefc09e6322da8a9321966e8928ef80d20f7f770f"
+
 [[package]]
 name = "lock_api"
 version = "0.4.12"
@@ -1546,7 +1651,7 @@ dependencies = [
  "supports-unicode",
  "terminal_size",
  "textwrap",
- "thiserror",
+ "thiserror 1.0.61",
  "unicode-width",
 ]
 
@@ -1558,7 +1663,7 @@ checksum = "49e7bc1560b95a3c4a25d03de42fe76ca718ab92d1a22a55b9b4cf67b3ae635c"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1647,6 +1752,7 @@ dependencies = [
  "fs_extra",
  "fxhash",
  "hex",
+ "insta",
  "itertools",
  "kimchi",
  "miette",
@@ -1661,7 +1767,7 @@ dependencies = [
  "serde",
  "serde_json",
  "serde_with 3.11.0",
- "thiserror",
+ "thiserror 1.0.61",
  "tokio",
  "toml",
  "tower",
@@ -1786,7 +1892,7 @@ dependencies = [
  "serde",
  "serde_with 1.14.0",
  "sha2",
- "thiserror",
+ "thiserror 1.0.61",
 ]
 
 [[package]]
@@ -1853,15 +1959,49 @@ checksum = "e3148f5046208a5d56bcfc03053e3ca6334e51da8dfb19b6cdc8b306fae3283e"
 
 [[package]]
 name = "pest"
-version = "2.7.10"
+version = "2.7.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "560131c633294438da9f7c4b08189194b20946c8274c6b9e38881a7874dc8ee8"
+checksum = "8b7cafe60d6cf8e62e1b9b2ea516a089c008945bb5a275416789e7db0bc199dc"
 dependencies = [
  "memchr",
- "thiserror",
+ "thiserror 2.0.6",
  "ucd-trie",
 ]
 
+[[package]]
+name = "pest_derive"
+version = "2.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "816518421cfc6887a0d62bf441b6ffb4536fcc926395a69e1a85852d4363f57e"
+dependencies = [
+ "pest",
+ "pest_generator",
+]
+
+[[package]]
+name = "pest_generator"
+version = "2.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7d1396fd3a870fc7838768d171b4616d5c91f6cc25e377b673d714567d99377b"
+dependencies = [
+ "pest",
+ "pest_meta",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.87",
+]
+
+[[package]]
+name = "pest_meta"
+version = "2.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1e58089ea25d717bfd31fb534e4f3afcc2cc569c70de3e239778991ea3b7dea"
+dependencies = [
+ "once_cell",
+ "pest",
+ "sha2",
+]
+
 [[package]]
 name = "pin-project-lite"
 version = "0.2.14"
@@ -1896,7 +2036,7 @@ dependencies = [
  "rmp-serde",
  "serde",
  "serde_with 1.14.0",
- "thiserror",
+ "thiserror 1.0.61",
 ]
 
 [[package]]
@@ -2041,7 +2181,7 @@ version = "0.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9b6dfecf2c74bce2466cabf93f6664d6998a69eb21e39f4207930065b27b771f"
 dependencies = [
- "bitflags",
+ "bitflags 2.5.0",
 ]
 
 [[package]]
@@ -2052,7 +2192,7 @@ checksum = "bd283d9651eeda4b2a83a43c1c91b266c40fd76ecd39a50a8c630ae69dc72891"
 dependencies = [
  "getrandom 0.2.15",
  "libredox",
- "thiserror",
+ "thiserror 1.0.61",
 ]
 
 [[package]]
@@ -2143,7 +2283,7 @@ dependencies = [
  "regex",
  "relative-path",
  "rustc_version 0.4.0",
- "syn 2.0.66",
+ "syn 2.0.87",
  "unicode-ident",
 ]
 
@@ -2196,6 +2336,15 @@ version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f3cb5ba0dc43242ce17de99c180e96db90b235b8a9fdc9543c96d2209116bd9f"
 
+[[package]]
+name = "same-file"
+version = "1.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93fc1dc3aaa9bfed95e02e6eadabb4baf7e3078b0bd1b4d7b6b0b68378900502"
+dependencies = [
+ "winapi-util",
+]
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"
@@ -2243,7 +2392,7 @@ checksum = "500cbc0ebeb6f46627f50f3f5811ccf6bf00643be300b4c3eabc0ef55dc5b5ba"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -2337,7 +2486,19 @@ dependencies = [
  "darling 0.20.9",
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
+]
+
+[[package]]
+name = "serde_yaml"
+version = "0.8.26"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "578a7433b776b56a35785ed5ce9a7e777ac0598aac5a6dd1b4b18a307c7fc71b"
+dependencies = [
+ "indexmap 1.9.3",
+ "ryu",
+ "serde",
+ "yaml-rust",
 ]
 
 [[package]]
@@ -2494,9 +2655,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.66"
+version = "2.0.87"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c42f3f41a2de00b01c0aaad383c5a45241efc8b2d1eda5661812fda5f3cdcff5"
+checksum = "25aa4ce346d03a6dcd68dd8b4010bcb74e54e62c90c573f394c46eae99aba32d"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -2531,6 +2692,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "termios"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "411c5bf740737c7918b8b1fe232dca4dc9f8e754b8ad5e20966814001ed0ac6b"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "textwrap"
 version = "0.15.2"
@@ -2548,7 +2718,16 @@ version = "1.0.61"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c546c80d6be4bc6a00c0f01730c08df82eaa7a7a61f11d656526506112cc1709"
 dependencies = [
- "thiserror-impl",
+ "thiserror-impl 1.0.61",
+]
+
+[[package]]
+name = "thiserror"
+version = "2.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8fec2a1820ebd077e2b90c4df007bebf344cd394098a13c563957d0afc83ea47"
+dependencies = [
+ "thiserror-impl 2.0.6",
 ]
 
 [[package]]
@@ -2559,7 +2738,18 @@ checksum = "46c3384250002a6d5af4d114f2845d37b57521033f30d5c3f46c4d70e1197533"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "2.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d65750cab40f4ff1929fb1ba509e9914eb756131cef4210da8d5d700d26f6312"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -2629,7 +2819,7 @@ checksum = "693d596312e88961bc67d7f1f97af8a70227d9f90c31bba5806eec004978d752"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -2701,7 +2891,7 @@ version = "0.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8437150ab6bbc8c5f0f519e3d5ed4aa883a83dd4cdd3d1b21f9482936046cb97"
 dependencies = [
- "bitflags",
+ "bitflags 2.5.0",
  "bytes",
  "futures-util",
  "http",
@@ -2842,6 +3032,16 @@ version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
 
+[[package]]
+name = "walkdir"
+version = "2.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "29790946404f91d9c5d06f9874efddea1dc06c5efe94541a7d6863108e3a5e4b"
+dependencies = [
+ "same-file",
+ "winapi-util",
+]
+
 [[package]]
 name = "wasi"
 version = "0.9.0+wasi-snapshot-preview1"
@@ -2875,7 +3075,7 @@ dependencies = [
  "once_cell",
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
  "wasm-bindgen-shared",
 ]
 
@@ -2897,7 +3097,7 @@ checksum = "e94f17b526d0a461a191c78ea52bbce64071ed5c04c9ffe424dcb38f74171bb7"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
  "wasm-bindgen-backend",
  "wasm-bindgen-shared",
 ]
@@ -2924,6 +3124,15 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
 
+[[package]]
+name = "winapi-util"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
+dependencies = [
+ "windows-sys",
+]
+
 [[package]]
 name = "winapi-x86_64-pc-windows-gnu"
 version = "0.4.0"
@@ -3030,6 +3239,15 @@ dependencies = [
  "tap",
 ]
 
+[[package]]
+name = "yaml-rust"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "56c1936c4cc7a1c9ab21a1ebb602eb942ba868cbd44a99cb7cdc5892335e1c85"
+dependencies = [
+ "linked-hash-map",
+]
+
 [[package]]
 name = "zeroize"
 version = "1.8.1"
@@ -3047,5 +3265,5 @@ checksum = "ce36e65b0d2999d2aafac989fb249189a141aee1f53c612c1f37d72631959f69"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.66",
+ "syn 2.0.87",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index d326d9a81..3c0bb03e9 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -55,3 +55,6 @@ tokio = { version = "1.41.0", features = ["full"] }
 tower = "0.5.1"
 tower-http = { version = "0.6.1", features = ["trace", "fs"] }
 tracing-subscriber = "0.3.18"
+
+[dev-dependencies]
+insta = { version =  "0.16", features = ["redactions",  "glob"] }
diff --git a/src/.DS_Store b/src/.DS_Store
new file mode 100644
index 000000000..51227d71a
Binary files /dev/null and b/src/.DS_Store differ
diff --git a/src/backends/r1cs/mod.rs b/src/backends/r1cs/mod.rs
index 6eb6527cd..8b1d4bd30 100644
--- a/src/backends/r1cs/mod.rs
+++ b/src/backends/r1cs/mod.rs
@@ -258,7 +258,7 @@ where
 }
 
 /// R1CS backend with bls12_381 field.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct R1CS<F>
 where
     F: BackendField,
diff --git a/src/error.rs b/src/error.rs
index b6ec94e24..f652c6a3a 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -342,6 +342,9 @@ pub enum ErrorKind {
     #[error("cannot compile a module without a main function")]
     NoMainFunction,
 
+    #[error("main function not found")]
+    MainFunctionNotFound,
+
     #[error("invalid hexadecimal literal `${0}`")]
     InvalidHexLiteral(String),
 
diff --git a/src/mast/mod.rs b/src/mast/mod.rs
index d955bc485..a2bc50c8e 100644
--- a/src/mast/mod.rs
+++ b/src/mast/mod.rs
@@ -467,11 +467,15 @@ pub fn monomorphize<B: Backend>(tast: TypeChecker<B>) -> Result<Mast<B>> {
     let mut ctx = MastCtx::new(tast);
 
     let qualified = FullyQualified::local("main".to_string());
-    let mut main_fn = ctx
-        .tast
-        .fn_info(&qualified)
-        .expect("main function not found")
-        .clone();
+
+    let mut main_fn = match ctx.tast.fn_info(&qualified) {
+        Some(fn_info) => fn_info.clone(),
+        None => Err(Error::new(
+            "Backend - Monomorphize",
+            ErrorKind::MainFunctionNotFound,
+            Span::default(),
+        ))?,
+    };
 
     let mut func_def = match &main_fn.kind {
         // `fn main() { ... }`
@@ -583,6 +587,8 @@ fn monomorphize_expr<B: Backend>(
             }
 
             // retrieve the function signature
+            dbg!(module);
+            dbg!(&fn_name.value);
             let old_qualified = FullyQualified::new(module, &fn_name.value);
             let mut fn_info = ctx
                 .tast
diff --git a/src/negative_tests.rs b/src/negative_tests.rs
index b50f86f46..ce117988b 100644
--- a/src/negative_tests.rs
+++ b/src/negative_tests.rs
@@ -50,7 +50,7 @@ fn tast_pass(code: &str) -> (Result<usize>, TypeChecker<R1csBackend>, Sources) {
     (res, tast, source)
 }
 
-fn mast_pass(code: &str) -> Result<Mast<R1csBackend>> {
+pub fn mast_pass(code: &str) -> Result<Mast<R1csBackend>> {
     let (_, tast, _) = tast_pass(code);
     crate::mast::monomorphize(tast)
 }
@@ -747,3 +747,57 @@ fn test_mut_cst_struct_field_prop() {
         ErrorKind::ArgumentTypeMismatch(..)
     ));
 }
+
+#[test]
+fn test_monomorphize_main_function_not_found() {
+    let code = r#"
+    fn some_fn(xx: Field) -> Field {
+        let yy = xx + 1;
+        return yy;
+    }
+    "#;
+
+    let res = mast_pass(code);
+    assert!(matches!(
+        res.unwrap_err().kind,
+        ErrorKind::MainFunctionNotFound
+    ));
+}
+
+#[test]
+fn test_monomorphize_duplicate_definition() {
+    let code = r#"
+    fn main(xx: Field) -> Field {
+        let xx = xx + 1;
+        let yy = xx + 1;
+        return yy;
+    }
+    "#;
+
+    let res = mast_pass(code);
+    let _xx = "xx".to_string();
+    assert!(matches!(
+        res.unwrap_err().kind,
+        ErrorKind::DuplicateDefinition(_xx),
+    ));
+}
+
+#[test]
+fn test_monomorphize_returntype_mismatch() {
+    let code = r#"
+    struct Thing {
+        val: Field,
+    }
+    fn main(xx: Field) -> Field {
+        let mut thing = Thing { val: 3 };
+        thing.val = xx;
+        return thing;
+    }
+    "#;
+
+    let res = mast_pass(code);
+    assert!(matches!(
+        res.unwrap_err().kind,
+        ErrorKind::ReturnTypeMismatch(..),
+    ));
+}
diff --git a/src/tests/.DS_Store b/src/tests/.DS_Store
new file mode 100644
index 000000000..5008ddfcf
Binary files /dev/null and b/src/tests/.DS_Store differ
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 3036fa55d..693838460 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -1,3 +1,4 @@
 mod examples;
 mod modules;
+mod snapshots;
 mod stdlib;
diff --git a/src/tests/snapshots.rs b/src/tests/snapshots.rs
new file mode 100644
index 000000000..10b93c5e2
--- /dev/null
+++ b/src/tests/snapshots.rs
@@ -0,0 +1,2 @@
+#[cfg(test)]
+mod mast;
diff --git a/src/tests/snapshots/mast.rs b/src/tests/snapshots/mast.rs
new file mode 100644
index 000000000..94068403d
--- /dev/null
+++ b/src/tests/snapshots/mast.rs
@@ -0,0 +1,305 @@
+use crate::negative_tests::mast_pass;
+
+fn test_monomorphization(code_snippet: &str) {
+    let mast = mast_pass(code_snippet).expect("Mast pass failed!");
+
+    // Test functions
+    for (fully_qualified, fn_info) in &mast.0.functions {
+        insta::assert_snapshot!(serde_json::to_string_pretty(fully_qualified).unwrap());
+        insta::assert_snapshot!(serde_json::to_string_pretty(fn_info).unwrap());
+    }
+
+    // Test node types
+    for (counter, ty_kind) in &mast.0.node_types {
+        insta::assert_snapshot!(serde_json::to_string_pretty(counter).unwrap());
+        insta::assert_snapshot!(serde_json::to_string_pretty(ty_kind).unwrap());
+    }
+
+    // Test structs/constants/node_id
+    insta::assert_snapshot!(serde_json::to_string_pretty(&mast.0.structs).unwrap());
+    insta::assert_snapshot!(serde_json::to_string_pretty(&mast.0.constants).unwrap());
+    insta::assert_snapshot!(serde_json::to_string_pretty(&mast.0.node_id).unwrap());
+}
+
+#[test]
+fn test_generic_array_access() {
+    let code = r#"
+    fn last(arr: [Field; LEN]) -> Field {
+        return arr[LEN - 1];
+    }
+    fn main(pub xx: Field) {
+        let array1 = [xx + 1, xx + 2, xx + 3];
+        let elm1 = last(array1);
+        assert_eq(elm1, 4);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_array_nested() {
+    let code = r#"
+    fn last(arr: [[Field; NN]; 3]) -> Field {
+        let mut newarr = [0; NN * 3];
+    
+        let mut index = 0;
+        for ii in 0..3 {
+            let inner = arr[ii];
+            for jj in 0..NN {
+                newarr[(ii * NN) + jj] = inner[jj];
+            }
+        }
+    
+        return newarr[(NN * 3) - 1];
+    }
+    
+    fn main(pub xx: Field) {
+        let array = [[xx + 1, xx + 2, xx + 3]; 3];
+        let elm = last(array);
+        assert_eq(elm, 4);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_builtin_bits() {
+    let code = r#"
+    use std::bits;
+
+    fn main(pub xx: Field) {
+        // calculate on a cell var
+        let bits = bits::to_bits(3, xx);
+        assert(!bits[0]);
+        assert(bits[1]);
+        assert(!bits[2]);
+
+        let val = bits::from_bits(bits);
+        assert_eq(val, xx);
+
+        // calculate on a constant
+        let cst_bits = bits::to_bits(3, 2);
+        assert(!cst_bits[0]);
+        assert(cst_bits[1]);
+        assert(!cst_bits[2]);
+
+        let cst = bits::from_bits(cst_bits);
+        assert_eq(cst, xx);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_fn_multi_init() {
+    let code = r#"
+    fn last(arr: [Field; LEN]) -> Field {
+        return arr[LEN - 1];
+    }
+    
+    fn main(pub xx: Field) {
+        let arr1 = [xx + 1, xx + 2];
+        let arr2 = [xx + 4, xx + 5];
+        let e1 = last(arr1); // 3
+        let e2 = last(arr2); // 6
+        assert_eq(e1 + e2, 9);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+// Optimized test for generic_for_loop.rs
+#[test]
+fn test_generic_for_loops() {
+    let code = r#"
+    struct Thing {
+        xx: Field,
+    }
+    
+    fn Thing.clone(self, const LEN: Field) -> [Field; LEN] {
+        let mut arr = [0; LEN];
+        for idx in 0..LEN {
+            arr[idx] = self.xx;
+        }
+        return arr;
+    }
+    
+    fn join(const LEN: Field, arr1: [Field; LLEN], arr2: [Field; RLEN]) -> [Field; LEN] {
+        let mut arr = [0; LEN];
+        for ii in 0..LLEN {
+            arr[ii] = arr1[ii];
+        }
+    
+        for jj in 0..RLEN {
+            arr[jj + LLEN] = arr2[jj];
+        }
+    
+        return arr;
+    }
+    
+    fn clone(const LEN: Field, val: Field) -> [Field; LEN] {
+        let mut arr = [0; LEN];
+        for idx in 0..LEN {
+            arr[idx] = val;
+        }
+        return arr;
+    }
+    
+    fn accumulate_mut(const INIT: Field) -> Field {
+        // it shouldn't fold these variables, even they hold constant values
+        // it should only fold the generic vars
+        let mut zz = INIT;
+        for ii in 0..3 {
+            zz = zz + zz;
+        }
+        return zz;
+    }
+    
+    fn main(pub xx: Field) {
+        let arr1 = [xx + 1, xx + 2];
+        let arr2 = [xx + 3, xx + 4];
+        
+        let arr = join(4, arr1, arr2);
+    
+        assert_eq(arr[0], arr1[0]);
+        assert_eq(arr[1], arr1[1]);
+        assert_eq(arr[2], arr2[0]);
+        assert_eq(arr[3], arr2[1]);
+    
+        // test that the generic function is callable within a for loop
+        let mut arr3 = [[0; 2]; 2];
+        for idx in 0..2 {
+            let cloned = clone(2, idx);
+            arr3[idx] = cloned;
+        }
+    
+        // test that the generic method is callable within a for loop
+        let thing = Thing { xx: 5 };
+        for idx in 0..2 {
+            let cloned = thing.clone(2);
+            arr3[idx] = cloned;
+        }
+    
+        let init_val = 1;
+        let res = accumulate_mut(init_val);
+        assert_eq(res, 8);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_iterator() {
+    let code = r#"
+    fn zip(a1: [Field; LEN], a2: [Field; LEN]) -> [[Field; 2]; LEN] {
+        let mut result = [[0; 2]; LEN];
+        for index in 0..LEN {
+            result[index] = [a1[index], a2[index]];
+        }
+        return result;
+    }
+    
+    fn main(pub arr: [Field; 3]) -> Field {
+        let expected = [1, 2, 3];
+        for pair in zip(arr, expected) {
+            assert_eq(pair[0], pair[1]);
+        }
+    
+        return arr[0];
+    }    
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_method_multi_init() {
+    let code = r#"
+    struct Thing {
+        xx: Field,
+    }
+    
+    fn Thing.gen(self, const LEN: Field) -> [Field; LEN] {
+        return [self.xx; LEN];
+    }
+    
+    fn main(pub xx: Field, yy: Field) {
+        let thing1 = Thing { xx: xx };
+        let arr1 = thing1.gen(2);
+        let arr2 = thing1.gen(3); 
+        assert_eq(arr1[1], arr2[2]);
+    
+        let thing2 = Thing { xx: yy };
+        let arr3 = thing2.gen(2);
+        assert_eq(arr3[1], arr2[2] + 1);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_nested_func() {
+    let code = r#"
+    fn nested_func(const LEN: Field) -> [Field; LEN] {
+        return [0; LEN];
+    }
+    fn mod_arr(val: Field) -> [Field; 3] {
+        let mut result = nested_func(3);
+        for idx in 0..3 {
+            result[idx] = val;
+        }
+        return result;
+    }
+    fn main(pub val: Field) {
+        let result = mod_arr(val);
+        assert_eq(result[0], val);
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_nested_method() {
+    let code = r#"
+    struct Thing {
+        xx: Field,
+    }
+    
+    fn Thing.nested_func(const LEN: Field) -> [Field; LEN] {
+        return [0; LEN];
+    }
+    
+    fn Thing.mod_arr(self) -> [Field; 3] {
+        // this generic function should be instantiated
+        let mut result = self.nested_func(3);
+        for idx in 0..3 {
+            result[idx] = self.xx;
+        }
+        return result;
+    }
+    
+    fn main(pub val: Field) -> [Field; 3] {
+        let thing = Thing {xx: val};
+        return thing.mod_arr();
+    }
+    "#;
+    test_monomorphization(code);
+}
+
+#[test]
+fn test_generic_repeated_array() {
+    let code = r#"
+    fn init_arr(const LEFT: Field) -> [Field; 1 + (LEFT * 2)] {
+        let arr = [0; 1 + (LEFT * 2)];
+        return arr;
+    }
+    
+    fn main(pub public_input: Field) -> [Field; 3] {
+        let mut arr = init_arr(1);
+        for ii in 0..3 {
+            arr[ii] = public_input;
+        }
+        return arr;
+    }
+    "#;
+    test_monomorphization(code);
+}
diff --git a/src/type_checker/mod.rs b/src/type_checker/mod.rs
index b904feb5b..047c3804a 100644
--- a/src/type_checker/mod.rs
+++ b/src/type_checker/mod.rs
@@ -96,6 +96,7 @@ where
     /// Mapping from node id to TyKind.
     /// This can be used by the circuit-writer when it needs type information.
     // TODO: I think we should get rid of this if we can
+    #[serde_as(as = "Vec<(_, _)>")]
     pub node_types: HashMap<usize, TyKind>,
 
     /// The last node id for the MAST phase to reference.