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Add example desugaring of contract key syntax (#465)

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* add example desugaring of contract key syntax

* Update ghc-lib/template-desugaring.md
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Shayne Fletcher 2019-04-13 10:04:30 -04:00 committed by GitHub
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@ -23,9 +23,9 @@ template Iou
regulators : [Party]
where
ensure amount > 0
signatory [issuer, owner]
signatory issuer, owner
observer regulators
agreement issuer ++ " will pay " ++ owner ++ " " ++ (show amount)
agreement issuer <> " will pay " <> owner <> " " <> (show amount)
choice Transfer : ContractId Iou
with
@ -68,9 +68,9 @@ Next, an `instance` declaration for `Iou` to declare its membership in `Template
```haskell
instance Template Iou where
ensure this@Iou{..} = amount > 0.0
signatory this@Iou{..} = [issuer, owner]
observer this@Iou{..} = [owner] ++ regulators
agreement this@Iou{..} = issuer ++ " will pay " ++ owner ++ " " ++ (show amount)
signatory this@Iou{..} = concat [toParties issuer, toParties owner]
observer this@Iou{..} = concat [toParties owner, toParties regulators]
agreement this@Iou{..} = issuer <> " will pay " <> owner <> " " <> (show amount)
```
When a type `c` is a `Template` instance, `class Choice` (defined by the DAML standard library) defines a (multi-parameter type class) relation on types `c`, `e` and `r` such that `r` is uniquely determined by the pair `(c, e)`:
@ -113,9 +113,9 @@ template Iou
regulators : [Party]
where
ensure amount > 0
signatory [issuer, owner]
signatory issuer, owner
observer regulators
agreement issuer ++ " will pay " ++ owner ++ " " ++ (show amount)
agreement issuer <> " will pay " <> owner <> " " <> (show amount)
controller [owner] can
Transfer : ContractId Iou
with
@ -143,9 +143,9 @@ data Iou = Iou {
instance Template Iou where
ensure this@Iou{..} = amount > 0.0
signatory this@Iou{..} = [issuer, owner]
observer this@Iou{..} = [owner] ++ regulators
agreement this@Iou{..} = issuer ++ " will pay " ++ owner ++ " " ++ (show amount)
signatory this@Iou{..} = concat [toParties issuer, toParties owner]
observer this@Iou{..} = concat [toParties owner, toParties regulators]
agreement this@Iou{..} = issuer <> " will pay " <> owner <> " " <> (show amount)
```
The two choices lead to two `instance Choice Iou e r` declarations, one for each of the triples `(Iou, Split, (ContractID Iou, ContractID Iou))` and `(Iou, Transfer, ContractID Iou)`:
@ -167,3 +167,47 @@ instance Choice Iou Transfer (ContractId Iou) where
choiceController this@Iou{..} arg@Transfer{..} = [owner]
choice this@Iou{..} self arg@Transfer{..} = create this with owner = newOwner
```
### Example (3)
The next contract exercises the so-called "contract keys" feature of DAML. Contract key syntax desugars to `instance` declarations of the following typeclass.
```haskell
class Template c => TemplateKey c k | c -> k where
key : c ->
maintainer : c -> [Party]
```
In the following `Enrollment` contract, there are no choices but there are declarations of `key` and `maintainer`.
```haskell
data Course =
Course with
institution : Party
title : Text
deriving (Show, Eq)
data Registration =
Registration with
student : Party
course : Course
year : Int
deriving (Show, Eq)
template Enrollment
with
reg : Registration
where
signatory reg.student, reg.course.institution
key reg : Registration
maintainer reg.course.institution
```
What the above desugars to is shown below.
```haskell
data Course = ...
data Registration = ...
instance Template Enrollment where
signatory this@Enrollment{..} = concat [toParties reg.student, toParties reg.course.institution]
instance TemplateKey Enrollment Registration where
key this@Enrollment{..} = reg
maintainer this@Enrollment{..} = concat [toParties reg.course.institution]
```