Pretty Printer: A Useful Feature Few Rust Programmers Know About
Surprisingly few know about the built-in pretty-printer. In the book, there is only a short passage that mentions {:#?} in passing.
It aligns structs and enums based on nested positions and is automatically derived with Debug. Recently, I’m working with some custom AST. Here is the print out for println("{:#?}", typed_ast):
TyProgram(
[
TyUseStmt(
TyUseStmt {
mod_name: "conv",
imported_names: [
"Conv2d",
"Dropout2d",
"maxpool2d"
]
}
),
TyUseStmt(
TyUseStmt {
mod_name: "nonlin",
imported_names: [
"relu"
]
}
),
TyUseStmt(
TyUseStmt {
mod_name: "lin",
imported_names: [
"Linear"
]
}
),
TyNodeDecl(
TyNodeDecl {
name: "Mnist",
ty_sig: ([!6, !7, !8, !9] -> [!6, <10>])
}
),
TyWeightsDecl(
TyWeightsDecl {
name: "Mnist",
ty_sig: ([!6, !7, !8, !9] -> [!6, <10>]),
inits: [
TyWeightsAssign {
name: "conv1",
ty: '11,
mod_name: "Conv2d",
fn_ty: '10,
fn_name: "new",
fn_args: [
TyFnAppArg {
name: "in_ch",
arg: TyExpr {
items: TyInteger(
'12,
1
),
ty: '13
}
},
TyFnAppArg {
name: "out_ch",
arg: TyExpr {
items: TyInteger(
'14,
10
),
ty: '15
}
},
TyFnAppArg {
name: "kernel_size",
arg: TyExpr {
items: TyInteger(
'16,
5
),
ty: '17
}
}
]
},
TyWeightsAssign {
name: "conv2",
ty: '19,
mod_name: "Conv2d",
fn_ty: '18,
fn_name: "new",
fn_args: [
TyFnAppArg {
name: "in_ch",
arg: TyExpr {
items: TyInteger(
'20,
10
),
ty: '21
}
},
TyFnAppArg {
name: "out_ch",
arg: TyExpr {
items: TyInteger(
'22,
20
),
ty: '23
}
},
TyFnAppArg {
name: "kernel_size",
arg: TyExpr {
items: TyInteger(
'24,
5
),
ty: '25
}
}
]
},
TyWeightsAssign {
name: "dropout",
ty: '27,
mod_name: "Dropout2d",
fn_ty: '26,
fn_name: "new",
fn_args: [
TyFnAppArg {
name: "p",
arg: TyExpr {
items: TyFloat(
'28,
0.5
),
ty: '29
}
}
]
},
TyWeightsAssign {
name: "fc1",
ty: '31,
mod_name: "Linear",
fn_ty: '30,
fn_name: "new",
fn_args: []
},
TyWeightsAssign {
name: "fc2",
ty: '33,
mod_name: "Linear",
fn_ty: '32,
fn_name: "new",
fn_args: []
}
]
}
),
TyGraphDecl(
TyGraphDecl {
name: "Mnist",
ty_sig: ([!6, !7, !8, !9] -> [!6, <10>]),
fns: [
TyFnDecl {
name: "new",
fn_params: [],
fn_ty: '34,
param_ty: '35,
return_ty: ([!6, !7, !8, !9] -> [!6, <10>]),
func_block: TyBlock {
stmts: TyList(
[
TyStmt {
items: TyList(
[
TyExpr {
items: TyFnApp(
TyFnApp {
mod_name: Some(
"fc1"
),
name: "init_normal",
args: [
TyFnAppArg {
name: "std",
arg: TyExpr {
items: TyFloat(
'36,
1.0
),
ty: '37
}
}
],
ret_ty: '38
}
),
ty: '39
}
]
)
},
TyStmt {
items: TyList(
[
TyExpr {
items: TyFnApp(
TyFnApp {
mod_name: Some(
"fc2"
),
name: "init_normal",
args: [
TyFnAppArg {
name: "std",
arg: TyExpr {
items: TyFloat(
'40,
1.0
),
ty: '41
}
}
],
ret_ty: '42
}
),
ty: '43
}
]
)
}
]
),
ret: TyExpr {
items: TyIdent(
([!6, !7, !8, !9] -> [!6, <10>]),
"self"
),
ty: '44
}
}
},
TyFnDecl {
name: "forward",
fn_params: [
TyFnDeclParam {
name: "b",
ty_sig: [!6, !7, !8, !9]
}
],
fn_ty: '46,
param_ty: '47,
return_ty: [!6, <10>],
func_block: TyBlock {
stmts: TyList(
[]
),
ret: TyExpr {
items: TyFnApp(
TyFnApp {
mod_name: None,
name: "log_softmax",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: Some(
"self"
),
name: "fc2",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "relu",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "fc1",
args: [
TyFnAppArg {
name: "x",
arg: TyViewFn(
TyViewFn {
ty: [!6, <320>],
arg: TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "maxpool2d",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "dropout",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "conv2",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "maxpool2d",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "conv1",
args: [
TyFnAppArg {
name: "x",
arg: TyIdent(
'45,
"b"
)
}
],
ret_ty: '48
}
)
},
TyFnAppArg {
name: "kernel_size",
arg: TyExpr {
items: TyInteger(
'49,
2
),
ty: '50
}
}
],
ret_ty: '51
}
)
}
],
ret_ty: '52
}
)
}
],
ret_ty: '53
}
)
},
TyFnAppArg {
name: "kernel_size",
arg: TyExpr {
items: TyInteger(
'54,
2
),
ty: '55
}
}
],
ret_ty: '56
}
)
}
}
)
}
],
ret_ty: '57
}
)
}
],
ret_ty: '58
}
)
}
],
ret_ty: '59
}
)
},
TyFnAppArg {
name: "dim",
arg: TyExpr {
items: TyInteger(
'60,
1
),
ty: '61
}
}
],
ret_ty: '62
}
),
ty: '63
}
}
},
TyFnDecl {
name: "fc2",
fn_params: [
TyFnDeclParam {
name: "x",
ty_sig: '64
}
],
fn_ty: '65,
param_ty: '66,
return_ty: [!6, <10>],
func_block: TyBlock {
stmts: TyList(
[]
),
ret: TyExpr {
items: TyFnApp(
TyFnApp {
mod_name: None,
name: "relu",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "fc2",
args: [
TyFnAppArg {
name: "x",
arg: TyIdent(
'64,
"x"
)
}
],
ret_ty: '67
}
)
}
],
ret_ty: '68
}
),
ty: '69
}
}
},
TyFnDecl {
name: "test",
fn_params: [
TyFnDeclParam {
name: "x",
ty_sig: '70
}
],
fn_ty: '71,
param_ty: '72,
return_ty: [],
func_block: TyBlock {
stmts: TyList(
[]
),
ret: TyExpr {
items: TyFnApp(
TyFnApp {
mod_name: None,
name: "relu",
args: [
TyFnAppArg {
name: "x",
arg: TyFnApp(
TyFnApp {
mod_name: None,
name: "fc2",
args: [
TyFnAppArg {
name: "x",
arg: TyIdent(
'70,
"x"
)
}
],
ret_ty: '73
}
)
}
],
ret_ty: '74
}
),
ty: '75
}
}
}
]
}
)
]
)
Also, it is not mutually exclusive with impl Debug for individual components, for example:
impl Debug for Type {
fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
use self::Type::*;
match self {
Unit => write!(f, "()"),
INT => write!(f, "int"),
BOOL => write!(f, "bool"),
VAR(ref t_id) => write!(f, "'{:?}", t_id),
DIM(ref t_id) => write!(f, "!{:?}", t_id),
ResolvedDim(ref d) => write!(f, "<{}>", d),
FUN(ref p, ref r) => write!(f, "({:?} -> {:?})", p, r),
TSR(ref dims) => {
if dims.len() > 0 {
write!(f, "[")?;
for i in dims[0..dims.len() - 1].iter() {
write!(f, "{:?}, ", i)?;
}
write!(f, "{:?}]", dims[dims.len() - 1])
} else {
write!(f, "[]")
}
}
}
}
}
Conclusion
In this click bait article, I introduced the pretty printer flag that few Rust programmers know about.