Create README.md

nomdl/codegen/README.md

@@ -0,0 +1,61 @@
+To update the codegen there are some subtleties because the code depends on generated code.
+
+## Build a working version
+
+First step is to build a binary.
+
+```
+cd nomdl/codegen/
+go build
+```
+
+## Change templates
+
+Not much to say here but you can see the result without breaking things
+
+```
+./codegen --in=test/struct.noms
+```
+
+This generates `test.noms.go` in the current directory. Iterate until it looks correct.
+
+## Change system go files
+
+There are a few files that are generated that codegen itself depends on.
+
+1. `types/compound_blob_struct.noms.go`
+1. `datas/types.noms.go`
+
+Both of these can be updated by running `go generate` in their respective directories
+
+There is also one more file that is generated but it requires manual intervention
+
+### `types/package_set_of_ref.go`
+
+This one is generated from `types/package_set_of_ref.noms`. However, it uses the symbol
+`Package` to refer to a `types.Package`. Currently we have no convenient way to make this work
+out of the box. However, it is pretty straight forward to make it work.
+
+1. Open `nomdl/pkg/grammar.pg`
+2. Find `UInt64`
+3. At that line, add one more builtin type called `Package`.
+4. Run `go generate` `in nomdl/pkg`
+5. Run `go run ../nomdl/codegen/codegen.go --in=package_set_of_ref.noms` in `types/`.
+
+Here is the diff:
+
+```diff
+--- a/nomdl/pkg/grammar.peg
++++ b/nomdl/pkg/grammar.peg
+@@ -159,7 +159,7 @@ CompoundType <- `List` _ `(` _ t:Type _ `)` _ {
+        return types.MakeCompoundTypeRef(types.RefKind, t.(types.TypeRef)), nil
+ }
+
+-PrimitiveType <- p:(`UInt64` / `UInt32` / `UInt16` / `UInt8` / `Int64` / `Int32` / `Int16` / `Int8` / `Float64` / `Float32` / `Bool` / `String` / `Blob` / `Value` / `TypeRef`) {
++PrimitiveType <- p:(`UInt64` / `UInt32` / `UInt16` / `UInt8` / `Int64` / `Int32` / `Int16` / `Int8` / `Float64` / `Float32` / `Bool` / `String` / `Blob` / `Value` / `TypeRef` / `Package`) {
+        return types.MakePrimitiveTypeRefByString(string(p.([]uint8))), nil
+ }
+ ```
+ 
+ Once [#577](https://github.com/attic-labs/noms/issues/577) is fixed this will need no manual intervention.
+