Connection with GHC's Constraint Solver
The solver for the type nats is implemented as an
extra stage in GHC's constrraint solver (see TcInteract.thePipeline
).
The following modules contain most of the code relevant for the solver:

TcTypeNats
: The main solver machinery 
TcTypeNatsRules
: The rules used by the solver 
TcTYpeNatsEval
: Functions for direct evaluation on constants
Generating Evidence
The solver produces evidence (i.e., proofs) when computing new "given"
constraints, or when solving existing "wanted" constraints.
The evidence is constructed by applications of a set of predefined
rules. The rules are values of type TypeRep.CoAxiomRule
.
Conceptually, rules have the form:
name :: forall tyvars. assumptions => conclusion
The rules have the usual logical meaning: the variables are universally quantified, and the assumptions imply the concluson. As a concrete example, consider the rule for leftcancellation of addtion:
AddCanceL :: forall a b c d. (a + b ~ d, a + c ~ d) => b ~ c
The type CoAxiomRule
also supports infinte literalindexed families
of simple axioms using constructor CoAxiomTyLit
. These have the form:
name(l_1 .. l_n) :: conclusion
In this case conclusion
is an equation that contains no type variables
but may depend on the literals in the name of the family. For example,
the basic definitional axiom for addition, TcTypeNatsRules.axAddDef
,
uses this mechanism:
AddDef(2,3) :: 2 + 3 ~ 5
At present, the assumptions and conclusion of all rules are equations between types but this restriction is not important and could be lifted in the future.
The rules used by the solver are in module TcTypeNatsRules
.
The Solver
The entry point to the solver is TcTypeNats.typeNatStage
.
We start by examining the constraint to see if it is obviously unsolvable
(using function impossible
), and if so we stash it in the
constraintsolver's state and stop. Note that there is no assumption that
impossible
is complete, but it is important that it is sound, so
if impossible
returns True
, then the constraint is definitely unsolvable,
but if impossible
returns False
, then we don't know if the constraint
is solvable or not.
The rest of the stage proceeds depending on the type of constraint, as follows.
Given Constraints
Given constraints correspond to adding new assumptions that may be used
by the solver. We start by checking if the new constraint is trivial
(using function solve
). A constraint is considered to be trivial
if it matches an already existing constraint or a rule that is known
to the solver. Such given constraints are ignored because they do not
contribute new information. If the new given is nontrivial, then it
will be recorded to the inert set as a new fact, and we proceed
to "interact" it with existing givens, in the hope of computing additional
useful facts (function computeNewGivenWork
).
IMPORTANT: We assume that "given" constraints are processed before "wanted" ones. A new given constraint may be used to solve any existing wanted, so every time we added a new given to the inert set we should move all potentially solvable "wanted" constraint from the inert set back to the work queue. We DON'T do this, because it is quite inefficient: there is no obvious way to compute which "wanted"s might be affected, so we have to restart all of them!
The heart of the interaction is the function interactCt
, which
performs one step of "forward" reasoning. The idea is to compute
new constraints whose proofs are made by an application of a rule
to the new given, and some existing givens. These new constraints are
added as new work, to be processed further on the next iteration of
GHC's constraint solver.
Aside: when we compute the new facts, we check to see if any are obvious contradictions. This is not strictly necessary because they would be detected on the next iteration of the solver. However, by doing the check early we get slightly better error messages because we can report the original constraint as being unsolvable (it leads to a contradiction), which tends to be easier to relate to the original program. Of course, this is not completely foolproofit is still possible that a contradiction is detected at a later iteration. An alternative ideanot yet implementedwould be to examine the proof of a contradiction and extract the original constraints that lead to it in the first place.
Derived Constraints
Derived
constraints are facts that are implied by the constraints
in the inert set. They do not have complete proofs because
they may depend on proofs of as yet unsolved wanted constraints.
GHC does not associate any proof terms with derived constraints (to keep things simple?).
In the constraint solver, they are mostly used as "hints". For example,
consider the wanted constraint 5 + 3 ~ x
, where x
is a
free unification variable. These are the steps we'll take to solve
the constraint:
Rules:
Add_def(5,3) : 5 + 3 ~ 8
Add_fun : forall a b c1 c2. (a + b ~ c1, a + b ~ c2) => c1 ~ c2
1. Add to inert set:
[W] C: 5 + 3 ~ x
2. Generate new derived:
[D] Add_fun(C,Add_def) : x ~ 8 (proof discarded)
3. GHC uses this hint to improve and reconsider the wanted:
[W] C: 5 + 3 ~ 8
4. Solved:
[W] C = Add_def(5,3)
The typenat solver processes derived constraints in a similar fashion
to given constraints (computeNewDerivedWork
): it checks to see if they are trivially known
and, if not, then it tries to generate some additional derived constraints.
The main difference is that derived constraints can be interacted
with all existing constraints to produce new facts, while given
constraints only interact with other givens.
Wanted Constraints
The main purpose of the solver is to discharge wanted
constraints
(the purpose of processing given and derived constraints is to help
solve existing wanted goals). When we encounter a new wanted goals
we proceed as follows:

Try to solve the goal, using a few different strategies:

Try to see if it matches the conclusion of an iff rule (
solveIff
). Aassumptions of rule become new wanted work. 
Try to see if it matches an axiom exactly (
solve
) 
Try the ordering solver for
<=
goals (solveLeq
) 
Try to use a (possibly synthesized) assumption

If that didn't work:

Wanted is added to the inert set

Check to see if any of the existing wanteds in the inert set can be solved in terms of the new goal (
reExamineWanteds
) 
Generate new derived facts.
Using IFF Rules
These rules are used to replace a wanted constraint with a collection of logically equivalent wanted constraints. If a wanted constraint matches the head of one of these rules, than it is solved using the rules, and the we generate new wanted constraints for the rule's assumptions.
The following are important properties of IFF rules:
 They need to be sound (of course!)
 The assumptions need to be logically equivalent to the conclusion (i.e., they should not result in a harder problem to solve than the original goal).
 The assumptions need to be simpler from the point of view of the constraint solver (i.e., we shouldn't end up with the original goal after some stepsthis would lead to nontermination).
At present, IFF rules are used to define certain operators in terms of others. For example, this is the only rule for solving constraints about subtraction:
forall a b c. (a + b ~ c) => (c  a ~ b)
Using Axioms
Basic operators are defined with an infinite family of axiom schemes.
As we can't have these written as a long list (searching might never stop!),
we have some custom code that checks to see if a constraint might be
solvable using one of the definitional axioms (see solveWithAxiom
, byAxiom
).
Using the Order Model
Constraints about the ordering of typelevel numbers are kept in a datastructure
(LeqFacts
) which forms a ``model* of the information represented by the
constraints (in a similar fashion to how substitutions form a model for a
set of equations).
*
The purpose of the model is to eliminate redundant constraints, and to make
it easy to find proofs for queries of the form x <= y
. In practise,
of particular interest are questions such as 1 <= x
because these appear
as assumptions on a number of rules (e.g., cancellation of multiplication).
In the future, this model could also be used to implement an interval
analysis, which would compute intervals approximating the values of
variables.
TODO At present, this model is reconstructed every time it needs to be used,
which is a bit inefficient. Perhaps it'd be better to use this directly
as the representation of <=
constraints in the inert set.
The model is a directed acyclic graph, as follows:
 vertices: constants or variables (of kind
Nat
)  edges: the edge from
A
toB
is a proof thatA <= B
.
So, to find a proof of A <= B
, we insert A
and B
in the model,
and then look for a path from A
to B
. The proofs on the path
can be composed using the rule for transitivity of <=
to form the final proof.
When manipulating the model, we maintain the following "minimality"
invariant: there should be no direct edge between two vertices A
and B
, if there is a path that can already get us from A
to `B.
Here are some examples (with edges pointing upwards)
B B
\ / \
 C C D
/ \ /
A A
Invariant does not hold Invariant holds
The purpose of the invariant is to eliminate redundant information. Note, however, that it does not guarantee that there is a unique way to prove a goal.
Using Extended Assumptions
Another way to prove a goal is to look it up in the assumptions. If the goal matched an assumption exactly, then GHC would have already solved it in one of its previous stages of the constraint solver. However, due to the commutativity and associativity of some of the operators, it is possible to have goal that could be solved by assumption, only if the assumption was "massaged" a bit.
This "massaging" is implemented by the function widenAsmps
, which
extends the set of assumption by performing a bit of forward reasoning
using a limited set of rules. Typically, these are commutativity
an associativity rules, and the widenAsmps
function tries to complete
the set of assumptions with respect to these operations. For example:
assumptions: C: x + y ~ z
cur. goal: D: y + x ~ z
extended assumptions: C: x + y ~ z, Add_Comm(C) : y + x ~ z
solved: D = Add_Comm(C)
Note that the extended assumptions are very similar to derived constraints, except that we keep their proofs.
Reexamining Wanteds
If none of the strategies for solving a wanted constraint worked,
then the constraint is added to the inert set. Since we'd like to
keep the inert set minimal, we have to see if any of the existing
wanted constraints might be solvable in terms of the new wanted
(reExamineWanteds
).
It is good to keep the inert set minimal for the following reasons:
 Inferred types are nicer,
 It helps GHC to solve constraints by "inlining" (e.g., if we
have only a single constraint
x + y ~ z
, then we can eliminate it by replacing all occurrences ofz
withx + y
, however we can't do that if we ended up with two constraints `(x + y ~ z, y + x ~ z)).
We consider each (numeric) wanted constraint in the inert set and check if we can solve it in terms of the new wanted and all other wanteds. If so, then it is removed from the inert set, otherwise it stays there.
Note that we can't implement this by kicking out the existing wanted constraints and putting them back on the work queue, because this would lead to nontermination. Here is an example of how this might happen:
inert: [W] A : x <= 5
new: [W] B : y <= 5
Can't solve B, add to inert, kick out A
inert: [W] B : y <= 5
new: [W] A : x <= 5
Can't solve A, add to inert, kick out B...
... and we are back to the beginning.
Perhaps there is a way around this but, for the moment, we just reexamine the numeric wanteds locally, without going through the constraint solver pipeline.