How Can I Restrict the Usage of an F# Union Type to A Particular Option
I am teaching myself F#--For Fun and Profit!--and, while I've made some strides, I have run into a stumbling block with usage of algebraic types. Below is a JSON type that I coded to serialize an arbitrary JSON structure to a string. I am open to subjective comments on it's design and efficiency, of course, but I am mainly focussed on line 7:
type JSON =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSON list
| JObject of Map< JSON, JSON >
with
member this.Serialize =
let rec serialize ( element : JSON ) =
match element with
| JString str ->
""" + str + """
| JNumber num ->
num.ToString()
| JBool bln ->
bln.ToString().ToLower()
| JNull ->
"null"
| JArray ary ->
"[" + String.concat "," ( List.map serialize ary ) + "]"
| JObject obj ->
"{" + (
Map.fold (
fun state key value ->
state + ( match state with "" -> "" | _ -> "," )
+ ( serialize key )
+ ":"
+ ( serialize value ) ) "" obj ) + "}"
serialize( this )
Anyone familiar with JSON knows that a key/value pair of a JSON object should be keyed on a string, not just any JSON element/value. Is there a way to further restrict the first type parameter of the Map? These, of course, do not work:
type JSON =
... elided ...
| JObject of Map< JSON.JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< string, JSON >
Thanks.
generics f# algebraic-data-types discriminated-union
asked Nov 12 at 17:46
David Clark
345
add a comment |
I am teaching myself F#--For Fun and Profit!--and, while I've made some strides, I have run into a stumbling block with usage of algebraic types. Below is a JSON type that I coded to serialize an arbitrary JSON structure to a string. I am open to subjective comments on it's design and efficiency, of course, but I am mainly focussed on line 7:
type JSON =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSON list
| JObject of Map< JSON, JSON >
with
member this.Serialize =
let rec serialize ( element : JSON ) =
match element with
| JString str ->
""" + str + """
| JNumber num ->
num.ToString()
| JBool bln ->
bln.ToString().ToLower()
| JNull ->
"null"
| JArray ary ->
"[" + String.concat "," ( List.map serialize ary ) + "]"
| JObject obj ->
"{" + (
Map.fold (
fun state key value ->
state + ( match state with "" -> "" | _ -> "," )
+ ( serialize key )
+ ":"
+ ( serialize value ) ) "" obj ) + "}"
serialize( this )
Anyone familiar with JSON knows that a key/value pair of a JSON object should be keyed on a string, not just any JSON element/value. Is there a way to further restrict the first type parameter of the Map? These, of course, do not work:
type JSON =
... elided ...
| JObject of Map< JSON.JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< string, JSON >
Thanks.
generics f# algebraic-data-types discriminated-union
asked Nov 12 at 17:46
David Clark
345
2
JObject of Map< string, JSON >works on my computer...
– Ringil
Nov 12 at 18:33
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00
add a comment |
I am teaching myself F#--For Fun and Profit!--and, while I've made some strides, I have run into a stumbling block with usage of algebraic types. Below is a JSON type that I coded to serialize an arbitrary JSON structure to a string. I am open to subjective comments on it's design and efficiency, of course, but I am mainly focussed on line 7:
type JSON =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSON list
| JObject of Map< JSON, JSON >
with
member this.Serialize =
let rec serialize ( element : JSON ) =
match element with
| JString str ->
""" + str + """
| JNumber num ->
num.ToString()
| JBool bln ->
bln.ToString().ToLower()
| JNull ->
"null"
| JArray ary ->
"[" + String.concat "," ( List.map serialize ary ) + "]"
| JObject obj ->
"{" + (
Map.fold (
fun state key value ->
state + ( match state with "" -> "" | _ -> "," )
+ ( serialize key )
+ ":"
+ ( serialize value ) ) "" obj ) + "}"
serialize( this )
Anyone familiar with JSON knows that a key/value pair of a JSON object should be keyed on a string, not just any JSON element/value. Is there a way to further restrict the first type parameter of the Map? These, of course, do not work:
type JSON =
... elided ...
| JObject of Map< JSON.JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< string, JSON >
Thanks.
generics f# algebraic-data-types discriminated-union
asked Nov 12 at 17:46
David Clark
345
I am teaching myself F#--For Fun and Profit!--and, while I've made some strides, I have run into a stumbling block with usage of algebraic types. Below is a JSON type that I coded to serialize an arbitrary JSON structure to a string. I am open to subjective comments on it's design and efficiency, of course, but I am mainly focussed on line 7:
type JSON =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSON list
| JObject of Map< JSON, JSON >
with
member this.Serialize =
let rec serialize ( element : JSON ) =
match element with
| JString str ->
""" + str + """
| JNumber num ->
num.ToString()
| JBool bln ->
bln.ToString().ToLower()
| JNull ->
"null"
| JArray ary ->
"[" + String.concat "," ( List.map serialize ary ) + "]"
| JObject obj ->
"{" + (
Map.fold (
fun state key value ->
state + ( match state with "" -> "" | _ -> "," )
+ ( serialize key )
+ ":"
+ ( serialize value ) ) "" obj ) + "}"
serialize( this )
Anyone familiar with JSON knows that a key/value pair of a JSON object should be keyed on a string, not just any JSON element/value. Is there a way to further restrict the first type parameter of the Map? These, of course, do not work:
type JSON =
... elided ...
| JObject of Map< JSON.JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< JString, JSON >
...
type JSON =
... elided ...
| JObject of Map< string, JSON >
Thanks.
generics f# algebraic-data-types discriminated-union
generics f# algebraic-data-types discriminated-union
asked Nov 12 at 17:46
David Clark
345
asked Nov 12 at 17:46
David Clark
345
asked Nov 12 at 17:46
David Clark
345
asked Nov 12 at 17:46
David Clark
345
asked Nov 12 at 17:46
David Clark
345
345
2
JObject of Map< string, JSON >works on my computer...
– Ringil
Nov 12 at 18:33
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00
add a comment |
2
JObject of Map< string, JSON >works on my computer...
– Ringil
Nov 12 at 18:33
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00
2
2
JObject of Map< string, JSON > works on my computer...– Ringil
Nov 12 at 18:33
JObject of Map< string, JSON > works on my computer...– Ringil
Nov 12 at 18:33
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00
add a comment |
1 Answer
1
active
oldest
votes
Referencing another case identifier from within a discriminated union is not possible. From Discriminated Unions,
Syntax
[ attributes ]
type [accessibility-modifier] type-name =
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...]
| case-identifier2 [of [fieldname3 : ] type3 [ * [ fieldname4 : ] type4 ...]
[ member-list ]
This means that each case identifier must be of some type. A case identifier itself is not a type.
One way you could achieve the same functionality is by breaking the discriminated union into multiple discriminated unions:
type JSONKey =
| JString of string
type JSONValue =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSONValue list
| JObject of Map<JSONKey, JSONValue>
and then defining JSON as:
type JSON = Map<JSONKey, JSONValue>
Then, serialize would need to be changed to let rec serialize ( element : JSONValue )
and
serialize( this ) would need to be changed to serialize( JObject this ).
As @Ringil mentioned, Map<string, JSON> will work in this situation, but this is not too extensible/restrictive.
answered Nov 12 at 18:31
ljeabmreosn
573520
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
add a comment |
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1 Answer
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1 Answer
1
active
oldest
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active
oldest
votes
active
oldest
votes
Referencing another case identifier from within a discriminated union is not possible. From Discriminated Unions,
Syntax
[ attributes ]
type [accessibility-modifier] type-name =
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...]
| case-identifier2 [of [fieldname3 : ] type3 [ * [ fieldname4 : ] type4 ...]
[ member-list ]
This means that each case identifier must be of some type. A case identifier itself is not a type.
One way you could achieve the same functionality is by breaking the discriminated union into multiple discriminated unions:
type JSONKey =
| JString of string
type JSONValue =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSONValue list
| JObject of Map<JSONKey, JSONValue>
and then defining JSON as:
type JSON = Map<JSONKey, JSONValue>
Then, serialize would need to be changed to let rec serialize ( element : JSONValue )
and
serialize( this ) would need to be changed to serialize( JObject this ).
As @Ringil mentioned, Map<string, JSON> will work in this situation, but this is not too extensible/restrictive.
answered Nov 12 at 18:31
ljeabmreosn
573520
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
add a comment |
Referencing another case identifier from within a discriminated union is not possible. From Discriminated Unions,
Syntax
[ attributes ]
type [accessibility-modifier] type-name =
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...]
| case-identifier2 [of [fieldname3 : ] type3 [ * [ fieldname4 : ] type4 ...]
[ member-list ]
This means that each case identifier must be of some type. A case identifier itself is not a type.
One way you could achieve the same functionality is by breaking the discriminated union into multiple discriminated unions:
type JSONKey =
| JString of string
type JSONValue =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSONValue list
| JObject of Map<JSONKey, JSONValue>
and then defining JSON as:
type JSON = Map<JSONKey, JSONValue>
Then, serialize would need to be changed to let rec serialize ( element : JSONValue )
and
serialize( this ) would need to be changed to serialize( JObject this ).
As @Ringil mentioned, Map<string, JSON> will work in this situation, but this is not too extensible/restrictive.
answered Nov 12 at 18:31
ljeabmreosn
573520
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
add a comment |
Referencing another case identifier from within a discriminated union is not possible. From Discriminated Unions,
Syntax
[ attributes ]
type [accessibility-modifier] type-name =
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...]
| case-identifier2 [of [fieldname3 : ] type3 [ * [ fieldname4 : ] type4 ...]
[ member-list ]
This means that each case identifier must be of some type. A case identifier itself is not a type.
One way you could achieve the same functionality is by breaking the discriminated union into multiple discriminated unions:
type JSONKey =
| JString of string
type JSONValue =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSONValue list
| JObject of Map<JSONKey, JSONValue>
and then defining JSON as:
type JSON = Map<JSONKey, JSONValue>
Then, serialize would need to be changed to let rec serialize ( element : JSONValue )
and
serialize( this ) would need to be changed to serialize( JObject this ).
As @Ringil mentioned, Map<string, JSON> will work in this situation, but this is not too extensible/restrictive.
answered Nov 12 at 18:31
ljeabmreosn
573520
Referencing another case identifier from within a discriminated union is not possible. From Discriminated Unions,
Syntax
[ attributes ]
type [accessibility-modifier] type-name =
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...]
| case-identifier2 [of [fieldname3 : ] type3 [ * [ fieldname4 : ] type4 ...]
[ member-list ]
This means that each case identifier must be of some type. A case identifier itself is not a type.
One way you could achieve the same functionality is by breaking the discriminated union into multiple discriminated unions:
type JSONKey =
| JString of string
type JSONValue =
| JString of string
| JNumber of decimal
| JBool of bool
| JNull
| JArray of JSONValue list
| JObject of Map<JSONKey, JSONValue>
and then defining JSON as:
type JSON = Map<JSONKey, JSONValue>
Then, serialize would need to be changed to let rec serialize ( element : JSONValue )
and
serialize( this ) would need to be changed to serialize( JObject this ).
As @Ringil mentioned, Map<string, JSON> will work in this situation, but this is not too extensible/restrictive.
answered Nov 12 at 18:31
ljeabmreosn
573520
edited Nov 12 at 18:41
answered Nov 12 at 18:31
ljeabmreosn
573520
answered Nov 12 at 18:31
ljeabmreosn
573520
answered Nov 12 at 18:31
ljeabmreosn
573520
573520
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
add a comment |
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
2
2
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
The case identifier NOT being a type is a distinction that was lost on me at this juncture. I was more or less thinking of them as subtypes. I appreciate the clarification and suggested solution.
– David Clark
Nov 12 at 21:01
add a comment |
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2
JObject of Map< string, JSON >works on my computer...– Ringil
Nov 12 at 18:33
I'll look into using string as the key again. I was having issues in the serialization of JObject with that. Thanks.
– David Clark
Nov 12 at 21:00