fix: ui: fix display of syntax and semantic errors

Improve display of syntax and semantic errors.

Closes #281

stan/model.py

@@ -227,6 +227,7 @@ async def go():
                             sampling_output.write_line("<info>Sampling:</info> <error>Initialization failed.</error>")
                             raise RuntimeError("Initialization failed.")
                         raise RuntimeError(message)
+
                     resp = await client.get(f"/{fit_name}")
                     if resp.status != 200:
                         raise RuntimeError((resp.json())["message"])
@@ -470,7 +471,19 @@ async def go():
             resp = task.result()
 
             if resp.status != 201:
-                raise RuntimeError(resp.json()["message"])
+                match = re.search(r"""ValueError\(['"](.*)['"]\)""", resp.json()["message"])
+                if not match:  # unknown error, should not happen
+                    raise RuntimeError(resp.json()["message"])
+                exception_body = match.group(1).encode().decode("unicode_escape")
+                error_type_match = re.match(r"(Semantic|Syntax) error", exception_body)
+                if error_type_match:
+                    error_type = error_type_match.group(0)
+                    exception_body_without_first_line = exception_body.split("\n", 1)[1]
+                    building_output.write_line(f"<info>Building:</info> <error>{error_type}:</error>")
+                    building_output.write_line(f"<error>{exception_body_without_first_line}</error>")
+                    raise ValueError(error_type)
+                else:
+                    raise RuntimeError(exception_body)
             building_output.clear()
             if model_in_cache:
                 building_output.write("<info>Building:</info> found in cache, done.")

tests/test_build_exceptions.py

@@ -0,0 +1,17 @@
+"""Tests for build related exceptions."""
+import pytest
+
+import stan
+
+
+def test_semantic_error():
+    # wrong number of arguments to `uniform`
+    program_code = "parameters { real y; } model { y ~ uniform(10, 20, 30); }" ""
+    with pytest.raises(ValueError, match=r"Semantic error"):
+        stan.build(program_code, random_seed=1)
+
+
+def test_syntax_error():
+    program_code = "parameters { real y; } model { y ~ uniform(10| 20); }" ""
+    with pytest.raises(ValueError, match=r"Syntax error"):
+        stan.build(program_code, random_seed=1)

tests/test_build_program.py

@@ -14,13 +14,13 @@ def test_build_basic():
 
 def test_stanc_no_such_distribution():
     program_code = "parameters {real z;} model {z ~ no_such_distribution();}"
-    with pytest.raises(RuntimeError, match=r"Semantic error in"):
+    with pytest.raises(ValueError, match=r"Semantic error"):
         stan.build(program_code=program_code)
 
 
 def test_stanc_invalid_assignment():
     program_code = "parameters {real z;} model {z = 3;}"
-    with pytest.raises(RuntimeError, match=r"Semantic error in"):
+    with pytest.raises(ValueError, match=r"Semantic error"):
         stan.build(program_code=program_code)
 
 
@@ -34,5 +34,5 @@ def test_stanc_exception_semicolon():
         z ~ normal(0, 1);
         y ~ normal(0, 1);}
     """
-    with pytest.raises(RuntimeError, match=r"Syntax error in"):
+    with pytest.raises(ValueError, match=r"Syntax error"):
         stan.build(program_code=program_code)