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Update transactions.md

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Krasimir Angelov
2021-09-23 13:03:18 +02:00
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parent cfc1e15fcf
commit cb6d3c4a2d
1 changed files with 3 additions and 3 deletions
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src/runtime/c/doc/transactions.md
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@@ -25,12 +25,12 @@ main = do
gr <- readNGF "Example.ngf" gr <- readNGF "Example.ngf"
print (functionType gr "f") print (functionType gr "f")
gr2 <- modifyPGF gr $ do gr2 <- modifyPGF gr $ do
<... do all updates here by using t ...> -- do all updates here
print (functionType gr2 "f") print (functionType gr2 "f")
``` ```
Here `modifyPGF` allows us to do updates but the updates are performed on a freshly created clone of the grammar `gr`. The original grammar is never ever modified. After the changes the variable `gr2` is a reference to the new revision. While the transaction is in progress we cannot see from the currently changing revision, and therefore all read-only operations can remain pure. Only after the transaction is complete then we get to use `gr2` which would not change anymore. Here `modifyPGF` allows us to do updates but the updates are performed on a freshly created clone of the grammar `gr`. The original grammar is never ever modified. After the changes the variable `gr2` is a reference to the new revision. While the transaction is in progress we cannot see from the currently changing revision, and therefore all read-only operations can remain pure. Only after the transaction is complete then we get to use `gr2` which would not change anymore.
Note also that above I used `functionType` with its usual pure type: Note also that above `functionType` is used with its usual pure type:
```Haskell ```Haskell
functionType :: PGF -> Fun -> Type functionType :: PGF -> Fun -> Type
``` ```
@@ -39,7 +39,7 @@ This is safe since the API never exposes database revisions which are not comple
main = do main = do
gr <- readNGF "Example.ngf" gr <- readNGF "Example.ngf"
gr2 <- modifyPGF gr $ do gr2 <- modifyPGF gr $ do
<... do all updates here by using t ...> -- do all updates here
print (functionType gr "f", functionType gr2 "f") print (functionType gr "f", functionType gr2 "f")
``` ```
The last line prints the type of function `"f"` in both the old and the new revision. Both are still available. The last line prints the type of function `"f"` in both the old and the new revision. Both are still available.