io-ts-graphql-codegen

io-ts-graphql-codegen

This plugin for graphql code generator generates types and runtime codecs for a given GraphQL Schema using io-ts to give runtime type safety.

Missing features:

• Custom codecs for custom Scalars
• Name aliases
• FragmentSpread selections
• subscription operation types

Assumptions

• At least one Mutation will be present
• At least one Query will be present
• __typename will be generated on all types and selections automatically

Architecture

The general flow to generate the types is as follows:

• Parse the schema into an AST
• Filter to Type Definitions and run topological sort on it so that dependant types appear first (this is crucial to printing the codecs in the correct order).
• Convert sorted graph to an io-ts AST representation.
• Build io-ts representation of all selections backing the selections with the above io-ts AST.
• Build io-ts representation of selection variables again backed with the above io-ts AST.
• Print all helpers to perform nested selections on codecs.
• Pretty print all io-ts ASTs.

Limitations

• io-ts's Intersection type is bound to a minimum of 2 codecs and a maximum of 5. When performing selections and merging them with intersection types these bounds need to be honored. Currently this plugin only supports the merging of 25 codecs.