Basic type checker #6

2026-06-05T09:31:36Z

HEL commented

2026-06-05 09:31:36 +00:00

This PR implements a basic type checker. It handles simple python constructs such as:

literals (i.e. booleans, integers, floats, strings)
variable assignment (w/ and w/o type annotation)
variable references
binary operations (i.e. +, -, *, /, etc.)
comparisons (i.e. >, <, >=, <=, etc.)
logical operations (i.e. and, or)
if statements
ternary expression
function definition
function call
return statements
casting (i.e. cast(Type, expression))

Some types and operations are defined by default to match what Python provides (e.g. literal types and their operations). The checker also supports importing custom type definitions from Midas files.
Currently, type checking is strict: types must match exactly. This is especially true for function return types, which must all be the same in a given function, and ternary expressions, whose branches must be of the same type.
Subtyping and genericity will be added in a separate PR

The type checker generates a list of diagnostics (errors, warnings and infos), which can be highlighted in the source file using the following command:

uv run midas compile -l highlighted.html -t types.midas source.py

This PR implements a basic type checker. It handles simple python constructs such as: - literals (i.e. booleans, integers, floats, strings) - variable assignment (w/ and w/o type annotation) - variable references - binary operations (i.e. `+`, `-`, `*`, `/`, etc.) - comparisons (i.e. `>`, `<`, `>=`, `<=`, etc.) - logical operations (i.e. `and`, `or`) - `if` statements - ternary expression - function definition - function call - return statements - casting (i.e. `cast(Type, expression)`) Some types and operations are defined by default to match what Python provides (e.g. literal types and their operations). The checker also supports importing custom type definitions from Midas files. Currently, type checking is strict: types must match exactly. This is especially true for function return types, which must all be the same in a given function, and ternary expressions, whose branches must be of the same type. Subtyping and genericity will be added in a separate PR The type checker generates a list of diagnostics (errors, warnings and infos), which can be highlighted in the source file using the following command: ```shell uv run midas compile -l highlighted.html -t types.midas source.py ```

HEL self-assigned this 2026-06-05 09:31:36 +00:00

HEL added 74 commits 2026-06-05 09:31:37 +00:00

docs: create architecture diagram 3f6b650a4b

feat(checker): create basic type and operation structs 6bf91db757

fix(parser): add function body and all_args property 8304760fe0

feat(checker): add environment manager c82b41a4df

adapted from Pebble

feat(checker): add python scope resolver 6de54e1da1

adapted from Pebble

chore: add simple operation and type examples 146be72fd7

feat(checker): add midas context resolver a872938405

this is still very basic and only handle a few expressions
notably, it doesn't support generics, option types, conditions, predicates nor complex types

feat(checker): add basic checker f882eebaf5

still very basic but lays out the structure and help methods

feat(cli): integrate checker in compile command 4b62c78874

fix(checker): get literal types from context 928901ef9c

fix(parser): add location in all AST nodes 218b0c5b78

feat(checker): add diagnostics 7515716864

chore: add some operations in the example 1b078b832c

fix(cli): enable midas ast dump 5dc6903425

fix(parser): prevent duplicate properties in complex types 022aebf55b

feat(parser): add return statements 8906ac3db8

feat(checker): evaluate function definitions fd5399f50a

feat(parser): parse function param defaults and sinks 3f61f84e5a

feat(checker): map and check function call arguments d16e192a3a

chore: tidy e6375f1aa9

refactor(tests): extract tester base class 89ec63cb05

refactor(tests): make cases dir configurable in subclass 86e4763a12

tests: move midas parser tests in subfolder e2d5943517

fix(tests): handle new tests with no snapshot 9dd547d6c1

tests: add python parser tester c3722c7438

tests: add tests for python parser 95b218fbed

fix(checker): stabilize call error message 13c19db818

display missing arguments in a stable format, similar to how native Python does

tests: add checker tester fa7e56cb77

tests: add tests for type checker c29c0ed3ec

feat(parser):add cast expression 0a3216e07d

feat(checker): add cast expression 4a3363a3d6

doc(checker): add documentation to checker methods 4ddde364ed

docs(checker): add documentation to checker, resolvers, etc. 1a1b0e8e15

feat(cli): dump environment after compile 893e1ba190

fix(cli): add cast visitor method to highlighter 7eb1e13b70

feat(parser): desugar AugAssign statements 112ed0e816

refactor(checker): move builtins definition to separate file f2c61d24e2

fix: update vscode syntax 29f691e38a

feat(cli): add option to highlight diagnostics 86ad348b99

fix(cli): ignore repeated visit of types 5d4df7978b

feat(parser): add if statement ab0fa1de1a

feat(checker): handle comparisons 9d45163d9c

feat(checker): type check if statements 65164abadb

fix(checker): handle paths with no returns in functions 1b66a8553d

feat(cli): update highlighter 261fd47494

chore: add control flow example f4dc57cb96

tests(checker): add control flow test bd31713ab4

fix(checker): early define fully-typed function 4151f5373d

to handle simple recursion cases where the function has an explicit return type hint, the function must be defined before evaluating its body

fix(cli): improve diagnostic message popup dd126f2559

feat(parser): add ternary statement 55060bfecd

feat(checker): handle ternary expression bea3f399ad

feat: add new midas syntax example 505779310a

feat(parser)!: add new Midas AST nodes ccb17c7290

feat(parser)!: update Midas parser with new nodes b9f378de6f

feat(checker)!: resolve new types 7f3d74ee49

chore: tidy 35ceda99aa

feat(cli): update highlighter with new nodes d70137775f

feat(tests): update Midas serializer c8536e20d2

fix(checker): wrap type definitions in AliasType dee479def5

feat(cli): add format command 97b1ee8ab8

feat(parser): parse pass statement and None 2fd2071d40

feat(checker): handle logical expressions a4139d4652

fix(tests): serialize ternary expressions 1c5c418f1c

fix(tests): update tests with new syntax 029caf4526

chore: update examples 63a43d79dd

fix!: remove union type 828ec9a3fa

tests: remove union type 9a934fabfd

refactor(checker): rename methods 822a74acce

improve a couple methods names, namely evaluate → type_of and evaluate_block → process_block

feat(checker): remove custom midas import 74b297c89c

remove custom import statement (`midas.using`) in favor of passing type definition files as arguments to the checker

fix(cli): add types files argument 5d7c724bc8

fix(tests): remove custom imports 73b21789d5

fix: update midas syntax definitions f182312cd2

fix: remove outdated syntax definition ddcaebb51a

Merge pull request 'Improve syntax and types' (#5 ) from feat/improve-syntax-and-types into feat/basic-type-checker 19b9fdd623

Reviewed-on: #5

HEL merged commit 13d32d0d27 into main

2026-06-05 09:31:54 +00:00

HEL deleted branch feat/basic-type-checker

2026-06-05 09:31:54 +00:00

HEL referenced this issue from a commit

2026-06-05 09:31:55 +00:00

Merge pull request 'Basic type checker' (#6) from feat/basic-type-checker into main

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Reference: HEL/midas#6