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package funcs
import (
"strconv"
"github.com/zclconf/go-cty/cty"
"github.com/zclconf/go-cty/cty/convert"
"github.com/zclconf/go-cty/cty/function"
)
// MakeToFunc constructs a "to..." function, like "tostring", which converts
// its argument to a specific type or type kind.
//
// The given type wantTy can be any type constraint that cty's "convert" package
// would accept. In particular, this means that you can pass
// cty.List(cty.DynamicPseudoType) to mean "list of any single type", which
// will then cause cty to attempt to unify all of the element types when given
// a tuple.
func MakeToFunc(wantTy cty.Type) function.Function {
return function.New(&function.Spec{
Params: []function.Parameter{
{
Name: "v",
// We use DynamicPseudoType rather than wantTy here so that
// all values will pass through the function API verbatim and
// we can handle the conversion logic within the Type and
// Impl functions. This allows us to customize the error
// messages to be more appropriate for an explicit type
// conversion, whereas the cty function system produces
// messages aimed at _implicit_ type conversions.
Type: cty.DynamicPseudoType,
AllowNull: true,
},
},
Type: func(args []cty.Value) (cty.Type, error) {
gotTy := args[0].Type()
if gotTy.Equals(wantTy) {
return wantTy, nil
}
conv := convert.GetConversionUnsafe(args[0].Type(), wantTy)
if conv == nil {
// We'll use some specialized errors for some trickier cases,
// but most we can handle in a simple way.
switch {
case gotTy.IsTupleType() && wantTy.IsTupleType():
return cty.NilType, function.NewArgErrorf(0, "incompatible tuple type for conversion: %s", convert.MismatchMessage(gotTy, wantTy))
case gotTy.IsObjectType() && wantTy.IsObjectType():
return cty.NilType, function.NewArgErrorf(0, "incompatible object type for conversion: %s", convert.MismatchMessage(gotTy, wantTy))
default:
return cty.NilType, function.NewArgErrorf(0, "cannot convert %s to %s", gotTy.FriendlyName(), wantTy.FriendlyNameForConstraint())
}
}
// If a conversion is available then everything is fine.
return wantTy, nil
},
Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
// We didn't set "AllowUnknown" on our argument, so it is guaranteed
// to be known here but may still be null.
ret, err := convert.Convert(args[0], retType)
if err != nil {
// Because we used GetConversionUnsafe above, conversion can
// still potentially fail in here. For example, if the user
// asks to convert the string "a" to bool then we'll
// optimistically permit it during type checking but fail here
// once we note that the value isn't either "true" or "false".
gotTy := args[0].Type()
switch {
case gotTy == cty.String && wantTy == cty.Bool:
what := "string"
if !args[0].IsNull() {
what = strconv.Quote(args[0].AsString())
}
return cty.NilVal, function.NewArgErrorf(0, `cannot convert %s to bool; only the strings "true" or "false" are allowed`, what)
case gotTy == cty.String && wantTy == cty.Number:
what := "string"
if !args[0].IsNull() {
what = strconv.Quote(args[0].AsString())
}
return cty.NilVal, function.NewArgErrorf(0, `cannot convert %s to number; given string must be a decimal representation of a number`, what)
default:
return cty.NilVal, function.NewArgErrorf(0, "cannot convert %s to %s", gotTy.FriendlyName(), wantTy.FriendlyNameForConstraint())
}
}
return ret, nil
},
})
}
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