github.com/bowd/gqlgen@v0.7.2/docs/content/reference/scalars.md (about)

     1  ---
     2  linkTitle: Custom Scalars
     3  title: Using custom graphql types in golang
     4  description: Defining custom GraphQL scalar types using gqlgen
     5  menu: { main: { parent: 'reference' } }
     6  ---
     7  
     8  There are two different ways to implement scalars in gqlgen, depending on your need.
     9  
    10  
    11  ## With user defined types
    12  For user defined types you can implement the graphql.Marshal and graphql.Unmarshal interfaces and they will be called.
    13  
    14  ```go
    15  package mypkg
    16  
    17  import (
    18  	"fmt"
    19  	"io"
    20  	"strings"
    21  )
    22  
    23  type YesNo bool
    24  
    25  // UnmarshalGQL implements the graphql.Marshaler interface
    26  func (y *YesNo) UnmarshalGQL(v interface{}) error {
    27  	yes, ok := v.(string)
    28  	if !ok {
    29  		return fmt.Errorf("points must be strings")
    30  	}
    31  
    32  	if yes == "yes" {
    33  		*y = true
    34  	} else {
    35  		*y = false
    36  	}
    37  	return nil
    38  }
    39  
    40  // MarshalGQL implements the graphql.Marshaler interface
    41  func (y YesNo) MarshalGQL(w io.Writer) {
    42  	if y {
    43  		w.Write([]byte(`"yes"`))
    44  	} else {
    45  		w.Write([]byte(`"no"`))
    46  	}
    47  }
    48  ```
    49  
    50  and then in .gqlgen.yml point to the name without the Marshal|Unmarshal in front:
    51  ```yaml
    52  models:
    53    YesNo:
    54      model: github.com/me/mypkg.YesNo
    55  ```
    56  
    57  
    58  ## Custom scalars for types you don't control
    59  
    60  Sometimes you cant add methods to a type because its in another repo, part of the standard 
    61  library (eg string or time.Time). To do this we can build an external marshaler:
    62  
    63  ```go
    64  package mypkg
    65  
    66  import (
    67  	"fmt"
    68  	"io"
    69  	"strings"
    70  	
    71  	"github.com/99designs/gqlgen/graphql"
    72  )
    73  
    74  
    75  func MarshalMyCustomBooleanScalar(b bool) graphql.Marshaler {
    76  	return graphql.WriterFunc(func(w io.Writer) {
    77  		if b {
    78  			w.Write([]byte("true"))
    79  		} else {
    80  			w.Write([]byte("false"))
    81  		}
    82  	})
    83  }
    84  
    85  func UnmarshalMyCustomBooleanScalar(v interface{}) (bool, error) {
    86  	switch v := v.(type) {
    87  	case string:
    88  		return "true" == strings.ToLower(v), nil
    89  	case int:
    90  		return v != 0, nil
    91  	case bool:
    92  		return v, nil
    93  	default:
    94  		return false, fmt.Errorf("%T is not a bool", v)
    95  	}
    96  }
    97  ```
    98  
    99  and then in .gqlgen.yml point to the name without the Marshal|Unmarshal in front:
   100  ```yaml
   101  models:
   102    MyCustomBooleanScalar:
   103      model: github.com/me/mypkg.MyCustomBooleanScalar
   104  ```
   105  
   106  see the [example/scalars](https://github.com/99designs/gqlgen/tree/master/example/scalars) package for more examples.