package function import ( "fmt" "github.com/zclconf/go-cty/cty" ) // Function represents a function. This is the main type in this package. type Function struct { spec *Spec } // Spec is the specification of a function, used to instantiate // a new Function. type Spec struct { // Params is a description of the positional parameters for the function. // The standard checking logic rejects any calls that do not provide // arguments conforming to this definition, freeing the function // implementer from dealing with such inconsistencies. Params []Parameter // VarParam is an optional specification of additional "varargs" the // function accepts. If this is non-nil then callers may provide an // arbitrary number of additional arguments (after those matching with // the fixed parameters in Params) that conform to the given specification, // which will appear as additional values in the slices of values // provided to the type and implementation functions. VarParam *Parameter // Type is the TypeFunc that decides the return type of the function // given its arguments, which may be Unknown. See the documentation // of TypeFunc for more information. // // Use StaticReturnType if the function's return type does not vary // depending on its arguments. Type TypeFunc // Impl is the ImplFunc that implements the function's behavior. // // Functions are expected to behave as pure functions, and not create // any visible side-effects. // // If a TypeFunc is also provided, the value returned from Impl *must* // conform to the type it returns, or a call to the function will panic. Impl ImplFunc } // New creates a new function with the given specification. // // After passing a Spec to this function, the caller must no longer read from // or mutate it. func New(spec *Spec) Function { f := Function{ spec: spec, } return f } // TypeFunc is a callback type for determining the return type of a function // given its arguments. // // Any of the values passed to this function may be unknown, even if the // parameters are not configured to accept unknowns. // // If any of the given values are *not* unknown, the TypeFunc may use the // values for pre-validation and for choosing the return type. For example, // a hypothetical JSON-unmarshalling function could return // cty.DynamicPseudoType if the given JSON string is unknown, but return // a concrete type based on the JSON structure if the JSON string is already // known. type TypeFunc func(args []cty.Value) (cty.Type, error) // ImplFunc is a callback type for the main implementation of a function. // // "args" are the values for the arguments, and this slice will always be at // least as long as the argument definition slice for the function. // // "retType" is the type returned from the Type callback, included as a // convenience to avoid the need to re-compute the return type for generic // functions whose return type is a function of the arguments. type ImplFunc func(args []cty.Value, retType cty.Type) (cty.Value, error) // StaticReturnType returns a TypeFunc that always returns the given type. // // This is provided as a convenience for defining a function whose return // type does not depend on the argument types. func StaticReturnType(ty cty.Type) TypeFunc { return func([]cty.Value) (cty.Type, error) { return ty, nil } } // ReturnType returns the return type of a function given a set of candidate // argument types, or returns an error if the given types are unacceptable. // // If the caller already knows values for at least some of the arguments // it can be better to call ReturnTypeForValues, since certain functions may // determine their return types from their values and return DynamicVal if // the values are unknown. func (f Function) ReturnType(argTypes []cty.Type) (cty.Type, error) { vals := make([]cty.Value, len(argTypes)) for i, ty := range argTypes { vals[i] = cty.UnknownVal(ty) } return f.ReturnTypeForValues(vals) } // ReturnTypeForValues is similar to ReturnType but can be used if the caller // already knows the values of some or all of the arguments, in which case // the function may be able to determine a more definite result if its // return type depends on the argument *values*. // // For any arguments whose values are not known, pass an Unknown value of // the appropriate type. func (f Function) ReturnTypeForValues(args []cty.Value) (ty cty.Type, err error) { var posArgs []cty.Value var varArgs []cty.Value if f.spec.VarParam == nil { if len(args) != len(f.spec.Params) { return cty.Type{}, fmt.Errorf( "wrong number of arguments (%d required; %d given)", len(f.spec.Params), len(args), ) } posArgs = args varArgs = nil } else { if len(args) < len(f.spec.Params) { return cty.Type{}, fmt.Errorf( "wrong number of arguments (at least %d required; %d given)", len(f.spec.Params), len(args), ) } posArgs = args[0:len(f.spec.Params)] varArgs = args[len(f.spec.Params):] } for i, spec := range f.spec.Params { val := posArgs[i] if val.IsNull() && !spec.AllowNull { return cty.Type{}, NewArgErrorf(i, "must not be null") } // AllowUnknown is ignored for type-checking, since we expect to be // able to type check with unknown values. We *do* still need to deal // with DynamicPseudoType here though, since the Type function might // not be ready to deal with that. if val.Type() == cty.DynamicPseudoType { if !spec.AllowDynamicType { return cty.DynamicPseudoType, nil } } else if errs := val.Type().TestConformance(spec.Type); errs != nil { // For now we'll just return the first error in the set, since // we don't have a good way to return the whole list here. // Would be good to do something better at some point... return cty.Type{}, NewArgError(i, errs[0]) } } if varArgs != nil { spec := f.spec.VarParam for i, val := range varArgs { realI := i + len(posArgs) if val.IsNull() && !spec.AllowNull { return cty.Type{}, NewArgErrorf(realI, "must not be null") } if val.Type() == cty.DynamicPseudoType { if !spec.AllowDynamicType { return cty.DynamicPseudoType, nil } } else if errs := val.Type().TestConformance(spec.Type); errs != nil { // For now we'll just return the first error in the set, since // we don't have a good way to return the whole list here. // Would be good to do something better at some point... return cty.Type{}, NewArgError(i, errs[0]) } } } // Intercept any panics from the function and return them as normal errors, // so a calling language runtime doesn't need to deal with panics. defer func() { if r := recover(); r != nil { ty = cty.NilType err = errorForPanic(r) } }() return f.spec.Type(args) } // Call actually calls the function with the given arguments, which must // conform to the function's parameter specification or an error will be // returned. func (f Function) Call(args []cty.Value) (val cty.Value, err error) { expectedType, err := f.ReturnTypeForValues(args) if err != nil { return cty.NilVal, err } // Type checking already dealt with most situations relating to our // parameter specification, but we still need to deal with unknown // values. posArgs := args[:len(f.spec.Params)] varArgs := args[len(f.spec.Params):] for i, spec := range f.spec.Params { val := posArgs[i] if !val.IsKnown() && !spec.AllowUnknown { return cty.UnknownVal(expectedType), nil } } if f.spec.VarParam != nil { spec := f.spec.VarParam for _, val := range varArgs { if !val.IsKnown() && !spec.AllowUnknown { return cty.UnknownVal(expectedType), nil } } } var retVal cty.Value { // Intercept any panics from the function and return them as normal errors, // so a calling language runtime doesn't need to deal with panics. defer func() { if r := recover(); r != nil { val = cty.NilVal err = errorForPanic(r) } }() retVal, err = f.spec.Impl(args, expectedType) if err != nil { return cty.NilVal, err } } // Returned value must conform to what the Type function expected, to // protect callers from having to deal with inconsistencies. if errs := retVal.Type().TestConformance(expectedType); errs != nil { panic(fmt.Errorf( "returned value %#v does not conform to expected return type %#v: %s", retVal, expectedType, errs[0], )) } return retVal, nil } // ProxyFunc the type returned by the method Function.Proxy. type ProxyFunc func(args ...cty.Value) (cty.Value, error) // Proxy returns a function that can be called with cty.Value arguments // to run the function. This is provided as a convenience for when using // a function directly within Go code. func (f Function) Proxy() ProxyFunc { return func(args ...cty.Value) (cty.Value, error) { return f.Call(args) } } // Params returns information about the function's fixed positional parameters. // This does not include information about any variadic arguments accepted; // for that, call VarParam. func (f Function) Params() []Parameter { new := make([]Parameter, len(f.spec.Params)) copy(new, f.spec.Params) return new } // VarParam returns information about the variadic arguments the function // expects, or nil if the function is not variadic. func (f Function) VarParam() *Parameter { if f.spec.VarParam == nil { return nil } ret := *f.spec.VarParam return &ret }