stlc-infer
A small interpreter for the simply typed lambda calculus, with type inference.
It's pretty heavily commented. Try looking at some files in the src/Stlc directory!
Usage
This project is written in Haskell, and we're using Stack to manage all dependencies. If you already have Stack installed, you can run:
stack build
stack test
stack exec -- stlc-infer
This launches an interactive REPL. The syntax is similar to Haskell, but only has intro and elim forms for functions, numbers, booleans. (You can also glean the syntax from test/Spec.hs and src/Stlc/Parser.hs).
❯ stack exec -- stlc-infer
stlc> (\x -> x) 0
Term: Tapp (Tlam (<x> Tvar 0@0)) Tz
Type: Cvar t_2
Constraints: fromList [Constraint (Carrow (Cvar t1_1) (Cvar t1_1)) (Carrow Cnat (Cvar t_2))]
Solution: fromList [ExplSubst t_2 Cnat,ExplSubst t1_1 Cnat]
Principal type: Cnat
Tz
stlc> let f = \x -> x in f True
Term: Tlet (Tlam (<x> Tvar 0@0)) (<f> Tapp (Tvar 0@0) (Tbool True))
Type: Cvar t_3
Constraints: fromList [Constraint (Carrow (Cvar t1_2) (Cvar t1_2)) (Carrow Cbool (Cvar t_3))]
Solution: fromList [ExplSubst t_3 Cbool,ExplSubst t1_2 Cbool]
Principal type: Cbool
Tbool True
stlc>
TODO
It should pretty much work. But since this is kind of a little playground for me, these are things I'm going to try to also implement:
- Replace naturals with proper integers
- Allow type annotations from programmer
- Let-polymorphism (simple version)
- Unit test individual files, i.e.:
- actually test that things infer to the right type
- actually test that things evaluate to the right value
- Better type error messages with position information
- Get type of identifier (or even expression) at position
