Safe Haskell | None |
---|---|

Language | Haskell98 |

A constructor argument is forced if it appears as pattern variable in an index of the target.

For instance `x`

is forced in `sing`

and `n`

is forced in `zero`

and `suc`

:

data Sing {a}{A : Set a} : A -> Set where sing : (x : A) -> Sing x data Fin : Nat -> Set where zero : (n : Nat) -> Fin (suc n) suc : (n : Nat) (i : Fin n) -> Fin (suc n)

At runtime, forced constructor arguments may be erased as they can be
recovered from dot patterns. In the epic backend,
```
unsing : {A : Set} (x : A) -> Sing x -> A
unsing .x (sing x) = x
```

becomes
```
unsing x sing = x
```

and
```
proj : (n : Nat) (i : Fin n) -> Nat
proj .(suc n) (zero n) = n
proj .(suc n) (suc n i) = n
```

becomes
```
proj (suc n) zero = n
proj (suc n) (suc i) = n
```

Forcing is a concept from pattern matching and thus builds on the concept of equality (I) used there (closed terms, extensional) which is different from the equality (II) used in conversion checking and the constraint solver (open terms, intensional).

Up to issue 1441 (Feb 2015), the forcing analysis here relied on the wrong equality (II), considering type constructors as injective. This is unsound for Epic's program extraction, but ok if forcing is only used to decide which arguments to skip during conversion checking.

From now on, forcing uses equality (I) and does not search for forced
variables under type constructors. This may lose some savings during
conversion checking. If this turns out to be a problem, the old
forcing could be brought back, using a new modality `Skip`

to indicate
that this is a relevant argument but still can be skipped during
conversion checking as it is forced by equality (II).

# Documentation

addForcingAnnotations :: Type -> TCM Type Source #

Given the type of a constructor (excluding the parameters),
decide which arguments are forced.
Update the relevance info in the domains accordingly.
Precondition: the type is of the form `Γ → D vs`

and the `vs`

are in normal form.

class ForcedVariables a where Source #

Compute the pattern variables of a term or term-like thing.

forcedVariables :: a -> [Nat] Source #

ForcedVariables Term Source # | Assumes that the term is in normal form. |

(ForcedVariables a, Foldable t) => ForcedVariables (t a) Source # | |