Json in Fortran

Our first program will demonstrate JSON encoding and decoding in Fortran. Here’s the full source code:

program json_example
    use json_module
    use iso_fortran_env, only: error_unit
    implicit none

    type :: response1
        integer :: page
        character(len=:), allocatable :: fruits(:)
    end type response1

    type :: response2
        integer :: page
        character(len=:), allocatable :: fruits(:)
    end type response2

    type(json_core) :: json
    type(json_value), pointer :: p, p_tmp
    type(response1) :: res1
    type(response2) :: res2
    character(len=:), allocatable :: json_string
    integer :: i

    ! Encoding basic data types to JSON strings
    call json%initialize()
    call json%create_object(p, '')

    call json%add(p, 'boolean', .true.)
    call json%add(p, 'integer', 1)
    call json%add(p, 'real', 2.34)
    call json%add(p, 'string', 'gopher')

    call json%print(p, json_string)
    print *, trim(json_string)
    call json%destroy(p)

    ! Encoding arrays and objects
    call json%create_object(p, '')
    call json%create_array(p_tmp, 'fruits')
    call json%add(p_tmp, '', 'apple')
    call json%add(p_tmp, '', 'peach')
    call json%add(p_tmp, '', 'pear')
    call json%add(p, p_tmp)
    
    call json%print(p, json_string)
    print *, trim(json_string)
    call json%destroy(p)

    ! Encoding custom data types
    res1%page = 1
    allocate(res1%fruits(3))
    res1%fruits = ['apple', 'peach', 'pear']

    call json%create_object(p, '')
    call json%add(p, 'Page', res1%page)
    call json%create_array(p_tmp, 'Fruits')
    do i = 1, size(res1%fruits)
        call json%add(p_tmp, '', res1%fruits(i))
    end do
    call json%add(p, p_tmp)

    call json%print(p, json_string)
    print *, trim(json_string)
    call json%destroy(p)

    ! Decoding JSON data
    json_string = '{"num":6.13,"strs":["a","b"]}'
    call json%initialize()
    call json%parse(json_string, p)

    call json%get(p, 'num', res2%page)  ! Note: Fortran doesn't have a direct equivalent to Go's interface{}
    print *, 'num:', res2%page

    call json%get(p, 'strs[1]', json_string)
    print *, 'First string:', trim(json_string)

    call json%destroy(p)

    call json%finalize()
end program json_example

This Fortran program demonstrates basic JSON encoding and decoding operations using the json_module. Here’s a breakdown of what the program does:

We start by encoding basic data types (boolean, integer, real, and string) to JSON strings.
We then encode an array of strings as a JSON array.
We create a custom type response1 and encode it to JSON.
Finally, we demonstrate decoding a JSON string into Fortran variables.

To run this program, you would typically compile it with a Fortran compiler that supports the json_module. For example:

$ gfortran -I/path/to/json_module json_example.f90 -o json_example
$ ./json_example

Note that Fortran doesn’t have built-in JSON support like Go does. We’re using an external module (json_module) to handle JSON operations. The exact syntax and available operations may vary depending on the specific JSON library you’re using.

Also, Fortran doesn’t have a direct equivalent to Go’s interface{} type, so type safety is generally stricter. When decoding JSON, you typically need to know the expected types in advance.

This example provides a basic introduction to JSON handling in Fortran. For more advanced usage, you should refer to the documentation of the specific JSON library you’re using.

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