# [−][src]Crate minitt

Rust implementation of Mini-TT, a simple dependently-typed lambda calculus.

Reading the README is recommended before reading the documentation.

# Tutorial

Here's a brief summary of the complete type-checking process.

Since this implementation is actually a dialect or, a variation of the original one, I use *minitt*
to represent this implementation and *Mini-TT* for the original one.

Here's a "feature list" (only language features that affect type-checking are listed):

First, Mini-TT supports:

- Pi (dependent function type)/Sigma (dependent tuple type) types
- First-class sum types and case-split
- (Mutual) Recursion

Mini-TT does not support (while you may expect it to support):

- Dependent pattern matching (with unification)
- Meta variables, say, implicit arguments

Mini-TT does not, but minitt does support:

- Constant expressions with type signature completely inferred
- Universe levels and its subtyping
- Notice:
`1`

is of level 0,`Type0`

is of level 1

- Notice:
- Sum types' merging operation and subtyping
(like
`Sum { A }`

is a subtype of`Sum { A | B }`

)

Version 0.1.8 of minitt is basically a vanilla Mini-TT, several extensions are introduced in later versions.

For those who want to have a try on minitt: Please do notice that function application in minitt is right-associative, which is very-very (very-very-very-very) anti-intuitive. This is because the parser is implemented primarily for debugging the type-checker, it's not for general-purpose programming. If you want to write some real code, I recommend Voile, which has nicer syntax, meta variables and implicit parameter syntax, and a non-dependent version of row-polymorphism.

## Syntax Trees

Mini-TT has three syntax trees:

- Surface syntax tree, aka concrete syntax tree, representing open expressions that may have free variables
- Abstract syntax tree, aka values or terms, representing expressions
that are already type-checked. This implies well-typedness and contextual well-scopedness
- Values might be neutral values: these values represents variable bindings that are not free but not reducible, like a parameter, or an expression that cannot be reduced due to another neutral subexpression
- Values might be closures: surface syntax term + context + parameter bindings

- Normal form syntax tree, aka normal forms.
This is the output of the "read back" (aka "reify") functions
- Details are introduced later

Mini-TT has two "environments":

- One typing context (called
`Gamma`

in minitt), which is passed around only during type-checking - One evaluation context (called
`Telescope`

in minitt), which is passed along with the typing context but is also accessible during evaluation. It's also stored in closures (as captured environment)

When they're together, they are stored in `TCS`

,
which is short for "Type-Checking State".

## Type-Checking

Mini-TT supports inferring types of simple expressions like applications, variable references, etc. But not the case for even a bit more complicated structures, like lambdas.

`checkD`

$$ \textnormal{checkD}\quad \rho,\Gamma\vdash_l D\Rightarrow \Gamma' $$

Each program is a sequence of definitions, each definition comes with a type signature and a body expression. We check the definitions one by one, after checking each definition we add it to the context and check the rest. For recursive definitions, we generate a neutral value before actually checking it.

This part is trivial in Mini-TT, but minitt extended definitions with *prefix parameters*, which
are parameters present before the type signature and the body expression, resulting in a much more
complicated implementation.

`check`

$$ \textnormal{check}\quad \rho,\Gamma\vdash_l M\Leftarrow t $$

This is the so-called `instance of`

check, the function name in Mini-TT paper is `check`

.

All definitions in Mini-TT comes along with a type signature, Mini-TT tries to type-check the signature and then try to match the body expression with the signature, using some hard-coded patterns (relevant codes are in check/expr.rs), like if the type is a pi-type and the value is a lambda, then we go on checking their bodies and types with the parameter instantiated as a generated value then recursively check if the instantiated body expression is an instance of the pi-type's return type; if the type is a sum type and the value is a constructor call, then check if the constructor is present in the sum.

If all these hard-coded rules are not applicable, infer the expression type and perform a subtyping check. This rule is an extension. The subtyping check is basically doing some hard-coded comparisons as well.

If it still fails, read back the types into their normal forms and do a syntactic comparison.

`checkI`

$$ \textnormal{checkI}\quad \rho,\Gamma\vdash_l M\Rightarrow t $$

Try to infer the type of a given expression.

Mini-TT/minitt cannot infer types of lambdas due to it's undecidable in general.

`checkT`

$$ \textnormal{checkT}\quad \rho,\Gamma\vdash_l A $$

Check if an expression is a type expression, returns the type's level because only `Value`

have the
method `level()`

(while returning the level is the only way for
`check_type`

to preserve this information. If we return a value, we can call `.level()`

on the
value).

Use some hard-coded rules and fallback to `check(expr, Type)`

.

## Possible Extensions

Several extensions can be made apart from the improvements that have nothing to do with the core type theory. I'm listing all the possible extension, disregarding how hard can the implementation be.

- Indexed inductive families
- Dependent (co)pattern matching
- Overlapping pattern matching
- Props
- Without-K

- Quantitative type theory
- Linear type system
- Affine type system

- First-class cases and sums (already implemented in Voile!)
- Record polymorphism
- Extensible sums

- Cubical type theory
- Already implemented in another Mini-TT dialect: cubicaltt
- Cartesian model?
- De Morgan model?
- Higher-inductive types
- First-classify them?

- Termination check
- Guarded recursion (productivity)
- Sized types (implicit?)

## Modules

ast | Syntax: term, expression, context. |

check | Type checking: everything related to type-checking. |

eval | Reduction: eval and eval's friends. |

parser | Parser, from text to AST and a bunch of related tools. |

pretty | Pretty print utilities. |

## Macros

tce_unreachable | Records the source code location that the error occurs, just like |

tcs_borrow | Cannot be an implementation of |