1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139

use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use syn::{parse, parse_macro_input, DeriveInput};

/// Derive `TryFrom<HashMap<String, String>>` for a struct.
///
/// This macro will generate an implementation of `TryFrom<HashMap<String, String>>` for the annotated struct.
/// It will attempt to parse each field from the map, and return an error if any field is missing or cannot be parsed.
/// Fields of type Option<T> are supported, and will be set to None if the field is missing from the map.
///
/// Currently only supports structs with named fields that impl FromStr.
/// Accepting all types that implement serde::Deserialize is a future goal.
///
/// # Example
///
/// ```rust
/// use try_from_map::TryFromMap;
///
/// #[derive(TryFromMap, Debug)]
/// struct Foo {
///    a: i32,
///    b: f32,
///    c: Option<bool>,
/// }
///
///
/// let map = std::collections::HashMap::from([
///     ("a".to_string(), "42".to_string()),
///     ("b".to_string(), "3.14".to_string()),
/// ]);
///
/// let foo = Foo::try_from(map).unwrap();
///
/// println!("{:?}", foo);
///
/// assert_eq!(foo.a, 42);
/// assert_eq!(foo.b, 3.14);
/// assert_eq!(foo.c, None);
///
/// ```
#[proc_macro_derive(TryFromMap)]
pub fn derive_try_from_map(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let parsed = parse_macro_input!(input as DeriveInput);
    let output = _derive_try_from_map(parsed);

    match output {
        Ok(output) => output.into(),
        Err(e) => e.into_compile_error().into(),
    }
}

fn _derive_try_from_map(parsed: DeriveInput) -> syn::Result<TokenStream> {
    let struct_name = parsed.ident.clone();
    let fields = parse_fields(&parsed)?;

    let from_impl = generate_from_impl(&struct_name, &fields);

    Ok(quote! {
        #from_impl
    })
}

fn generate_from_impl(struct_name: &syn::Ident, fields: &[syn::Field]) -> TokenStream {
    let field_idents = fields.iter().map(|f| f.ident.clone().unwrap());

    let extract_fields = fields
        .iter()
        .map(|field_ident| {
            let ident = field_ident.ident.as_ref().unwrap();
            let ident_str = format!("{}", ident);

            if is_optional_field(field_ident) {
                return quote! {
                    let #ident = match map.get(#ident_str) {
                        Some(value) => Some(value.parse()?),
                        None => None,
                    }
                };
            }

            let err_msg = format!("Field {} not found", ident);

            quote! {
                let #ident = map.get(#ident_str).ok_or_else(|| #err_msg)?.parse()?
            }
        })
        .collect::<Vec<_>>();

    quote! {
        impl std::convert::TryFrom<std::collections::HashMap<String, String>> for #struct_name {
            type Error = std::boxed::Box<dyn std::error::Error>;

            fn try_from(map: std::collections::HashMap<String, String>) -> Result<Self, Self::Error> {
                #(#extract_fields;)*

                Ok(Self {
                    #(#field_idents,)*
                })
            }
        }
    }
}

fn parse_fields(parsed: &DeriveInput) -> syn::Result<Vec<syn::Field>> {
    match &parsed.data {
        syn::Data::Struct(data) => match &data.fields {
            syn::Fields::Named(fields) => Ok(fields.named.iter().cloned().collect()),
            _ => Err(syn::Error::new_spanned(
                parsed,
                "Only named fields are supported",
            )),
        },
        _ => Err(syn::Error::new_spanned(
            parsed,
            "Only structs are supported",
        )),
    }
}

fn is_optional_field(field: &syn::Field) -> bool {
    let path_segments = match &field.ty {
        syn::Type::Path(syn::TypePath {
            qself: None,
            path: syn::Path { segments, .. },
        }) => segments,
        _ => return false,
    };

    let segment = match path_segments.first() {
        Some(segment) => segment,
        None => return false,
    };

    if segment.ident != "Option" {
        return false;
    }

    true
}