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//!
//!  Node value type
use std::convert::TryFrom;

use proc_macro2::TokenStream;
use syn::{token::Brace, Block};

#[derive(Clone, Debug, syn_derive::ToTokens, syn_derive::Parse)]
pub struct InvalidBlock {
    #[syn(braced)]
    pub brace: Brace,
    #[syn(in = brace)]
    pub body: TokenStream,
}

/// Block node.
///
/// Arbitrary rust code in braced `{}` blocks.
#[derive(Clone, Debug, syn_derive::ToTokens)]
pub enum NodeBlock {
    /// The block value..
    ValidBlock(Block),

    Invalid(InvalidBlock),
}

impl NodeBlock {
    ///
    /// Returns syntactically valid `syn::Block` of Rust code.
    ///
    /// Usually to make macro expansion IDE friendly, its better to use
    /// `ToTokens` instead. Because it also emit blocks that is invalid for
    /// syn, but valid for rust and rust analyzer. But if you need early
    /// checks that this block is valid  - use this method.
    ///
    /// Example of blocks that will or will not parse:
    /// ```no_compile
    /// {x.} // Rust will parse this syntax, but for syn this is invalid Block, because after dot ident is expected.
    ///      // Emiting code like this for rust analyzer allows it to find completion.
    ///      // This block is parsed as NodeBlock::Invalid
    /// {]}   // this is invalid syntax for rust compiler and rust analyzer so it will not be parsed at all.
    /// {x + y} // Valid syn Block, parsed as NodeBlock::Valid
    /// ```
    pub fn try_block(&self) -> Option<&Block> {
        match self {
            Self::ValidBlock(b) => Some(b),
            Self::Invalid(_) => None,
        }
    }
}

impl TryFrom<NodeBlock> for Block {
    type Error = syn::Error;
    fn try_from(v: NodeBlock) -> Result<Block, Self::Error> {
        match v {
            NodeBlock::ValidBlock(v) => Ok(v),
            NodeBlock::Invalid(_) => Err(syn::Error::new_spanned(
                v,
                "Cant parse expression as block.",
            )),
        }
    }
}