doc_code_snippets! { mod "guide_06", type_ident=Guide06, template=r##" This chapter demonstrates how to mutate the ConstValue parameter with a Mutator Function. Mutator Functions are TypeFn which can mutate the ConstValue parameter of a type, declared using the mutator_fn macro. Here is an example of a builder which checks that all fields have been initialized at compile-time. //@use_codeblock:tetris_pieces_struct,ignore This is the type created by the builder. It is the ammound of tetrominos (tetris pieces) used for a special for of tetris where one must fill as many lines as possible before the pieces run out. //@use_codeblock:field_init_enum,ignore This is the enum describing whether a field is initialized. //@use_codeblock:init_struct,ignore This is the struct describing describing the initialization state of all fields. //@use_codeblock:init_aliases,ignore These are aliases for ConstInitializedFields where all the fields are either initialized (AllInitialized) or uninitialized (AllUninitialized). //@use_codeblock:builder_struct_decl,ignore This is the builder struct itself, taking the initialization of each field as the ConstValue-parameter `C`. //@use_codeblock:mutator_fn,ignore This declares a Mutator Function for TetrisBuilder, I is the current value of the ConstValue parameter,and Field is the field we want to initialize. The `AllowedSelf` associated type allows us to control which methods of MutConstParam we can call,by value/reference/mutable_reference/all. the `let` syntax here is used to declare a variable in the impl of TypeFn_ for InitializeField. //@use_codeblock:declare_setter_macro,ignore Here we use a macro to declare each setter method of the builder since it is such a repetitive task. The mutparam method comes from the MutConstParam trait and allows changing the ConstValue-parameter of a value. MCPBounds is a trait alias for the constraints required by most MutConstParam methods. The `NextSelf=_Out` is an example of the [`generic type as type alias` ](../../appendix_patterns/index.html#patterngeneric-type-as-type-alias) pattern, which allows us to return whatever the trait's associated type is without repeating the constraint in the return type. //@use_codeblock:setter_impls,ignore This decares the setter methods for each field in the builder. //@use_codeblock:build_fn,ignore This constructs the TetrisPieces,the unwraps are fine since this type already checks whether all the fields are initialized with the ConstValue-parameter. The `C:TypeIdentity` constraint in the method is an equality constraint,requiring that the ConstValue-parameter be AllInitialized, and produces a better error message than if the impl was for TetrisBuilder< AllInitialized >. //@use_codeblock:main,ignore This looks like a typical builder type. if any of the setters are commented out this will result in a compiletime error.



















# The entire thing //@use_codeblock:all,rust "##, code=r##" //@codeblock-start:all #[macro_use] extern crate derive_type_level; #[macro_use] extern crate type_level_values; use type_level_values::prelude::*; use type_level_values::field_traits::{SetField,SetField_}; //@codeblock-start:tetris_pieces_struct #[derive(Clone, Debug,PartialEq)] pub struct TetrisPieces{ l_pieces:usize, i_pieces:usize, z_pieces:usize, s_pieces:usize, o_pieces:usize, } //@codeblock-end :tetris_pieces_struct //@codeblock-start:field_init_enum #[derive(TypeLevel)] #[typelevel( derive(ConstEq,ConstOrd), reexport(Variants), )] pub enum FieldInitialization{ InitField, UninitField, } //@codeblock-end :field_init_enum //@codeblock-start:init_struct #[derive(TypeLevel)] #[typelevel( derive(ConstEq,ConstOrd), reexport(Struct,Traits), )] pub struct InitializedFields{ pub l_pieces:FieldInitialization, pub i_pieces:FieldInitialization, pub z_pieces:FieldInitialization, pub s_pieces:FieldInitialization, pub o_pieces:FieldInitialization, } pub use self::type_level_InitializedFields::fields as if_f; //@codeblock-end :init_struct //@codeblock-start:init_aliases pub type AllUninitialized=SetField< InitializedFields_Uninit, if_f::All, UninitField >; pub type AllInitialized=SetField< InitializedFields_Uninit, if_f::All, InitField >; //@codeblock-end:init_aliases //@codeblock-start:builder_struct_decl #[derive(MutConstValue)] #[mcv( derive(Clone,Default, Debug), Type = "TetrisBuilder",ConstValue = "C", )] pub struct __TetrisBuilder{ l_pieces:Option, i_pieces:Option, z_pieces:Option, s_pieces:Option, o_pieces:Option, initialization:ConstWrapper, } impl TetrisBuilder< AllUninitialized >{ fn new()->Self{ TetrisBuilder::default() } } //@codeblock-end:builder_struct_decl //@codeblock-start:mutator_fn mutator_fn!{ type This[I]=(TetrisBuilder) type AllowedSelf=(allowed_self_constructors::ByVal) fn InitializeField[I,Field](I,Field) where [ I:SetField_, ] { let Out;Out } } //@codeblock-end :mutator_fn //@codeblock-start:declare_setter_macro macro_rules! declare_setter { ( $field:ident ) => ( fn $field<_Out>(mut self,value:usize)->_Out where Self:MCPBounds, { self.$field=Some(value); self.mutparam(InitializeField::NEW,if_f::$field::T) } ) } //@codeblock-end :declare_setter_macro //@codeblock-start:setter_impls impl TetrisBuilder< C >{ declare_setter!{ l_pieces } declare_setter!{ i_pieces } declare_setter!{ z_pieces } declare_setter!{ s_pieces } declare_setter!{ o_pieces } } //@codeblock-end:setter_impls //@codeblock-start:build_fn impl TetrisBuilder< C >{ fn build(self)->TetrisPieces where C:TypeIdentity { TetrisPieces{ l_pieces:self.l_pieces.unwrap(), i_pieces:self.i_pieces.unwrap(), z_pieces:self.z_pieces.unwrap(), s_pieces:self.s_pieces.unwrap(), o_pieces:self.o_pieces.unwrap(), } } } //@codeblock-end:build_fn fn main(){ //@codeblock-start:main let pieces=TetrisBuilder::new() .l_pieces(10) .i_pieces(20) .z_pieces(30) .s_pieces(40) .o_pieces(50) .build(); assert_eq!( pieces, TetrisPieces{ l_pieces:10, i_pieces:20, z_pieces:30, s_pieces:40, o_pieces:50, } ) //@codeblock-end :main } "##, }