On Thursday, 14 March 2013 at 17:07:16 UTC, Andrei Alexandrescu
wrote:
Very simple. Traditionally there's two crucial epochs known as
compilation time and run time. (There's some minor distinctions
like link time etc.) The whole notion of concepts and other
type systems for templates is predicated on three crucial
epochs: library compilation time, library user compilation
time, and run time. The logic goes, someone writes a generic
library and wants to distribute it to users. Users shouldn't
ever see bugs caused by e.g. typos in the library.
I'm not sure if you are thinking I'm really stupid here.
So the crowd that use meta-type systems is formed of library
writers who want to distribute libraries without ever
instantiating them. I don't think that's a good crowd to cater
for.
I've been surprised to figure how many people don't get this
flow, or only have a vague image of it. Although meta-types are
arguably "the right thing" to do, they're a lot less attractive
once it's clear what scenarios they help.
Let me demonstrate with an actual example :
libd-llvm/libd/src/d/parser/ambiguous.d(126): Error: no property
'bar' for type 'Lexer', did you mean 'r'?
libd-llvm/libd/src/d/parser/statement.d(392): Error: template
instance
d.parser.statement.parseStatement!(Lexer).parseStatement.parseDeclarationOrExpression!(__dgliteral27,
Lexer) error instantiating
libd-llvm/libd/src/d/parser/statement.d(406): instantiated
from here: parseStatement!(Lexer)
libd-llvm/libd/src/d/parser/expression.d(652):
instantiated from here: parseBlock!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(208): instantiated
from here: parsePrimaryExpression!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(83): ... (11
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/statement.d(406): Error: template
instance d.parser.statement.parseStatement!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/expression.d(652):
instantiated from here: parseBlock!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(208): instantiated
from here: parsePrimaryExpression!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(83): instantiated
from here: parseTemplateArguments!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(22): ... (10
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/expression.d(652): Error: template
instance d.parser.statement.parseBlock!(Lexer) error instantiating
libd-llvm/libd/src/d/parser/dtemplate.d(208): instantiated
from here: parsePrimaryExpression!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(83): instantiated
from here: parseTemplateArguments!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(22): instantiated
from here: parseBuiltIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(56): ... (9
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(208): Error: template
instance d.parser.expression.parsePrimaryExpression!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/identifier.d(83): instantiated
from here: parseTemplateArguments!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(22): instantiated
from here: parseBuiltIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(56): instantiated from
here: parseIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(16): ... (8
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(83): Error: template
instance d.parser.dtemplate.parseTemplateArguments!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/identifier.d(22): instantiated
from here: parseBuiltIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(56): instantiated from
here: parseIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(16): instantiated from
here: parseBasicType!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(112): ... (7
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(22): Error: template
instance d.parser.identifier.parseBuiltIdentifier!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/type.d(56): instantiated from
here: parseIdentifier!(Lexer)
libd-llvm/libd/src/d/parser/type.d(16): instantiated from
here: parseBasicType!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(112): instantiated
from here: parseType!(cast(ParseMode)0, Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(68): ... (6
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/type.d(56): Error: template instance
d.parser.identifier.parseIdentifier!(Lexer) error instantiating
libd-llvm/libd/src/d/parser/type.d(16): instantiated from
here: parseBasicType!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(112): instantiated
from here: parseType!(cast(ParseMode)0, Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(68): instantiated
from here: parseTypeParameter!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(46): ... (5
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/type.d(16): Error: template instance
d.parser.type.parseBasicType!(Lexer) error instantiating
libd-llvm/libd/src/d/parser/dtemplate.d(112): instantiated
from here: parseType!(cast(ParseMode)0, Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(68): instantiated
from here: parseTypeParameter!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(46): instantiated
from here: parseTemplateParameter!(Lexer)
libd-llvm/libd/src/d/parser/dfunction.d(51): ... (4
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(112): Error: template
instance d.parser.type.parseType!(cast(ParseMode)0, Lexer) error
instantiating
libd-llvm/libd/src/d/parser/dtemplate.d(68): instantiated
from here: parseTypeParameter!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(46): instantiated
from here: parseTemplateParameter!(Lexer)
libd-llvm/libd/src/d/parser/dfunction.d(51): instantiated
from here: parseTemplateParameters!(Lexer)
libd-llvm/libd/src/d/parser/declaration.d(362): ... (3
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(68): Error: template
instance d.parser.dtemplate.parseTypeParameter!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/dtemplate.d(46): instantiated
from here: parseTemplateParameter!(Lexer)
libd-llvm/libd/src/d/parser/dfunction.d(51): instantiated
from here: parseTemplateParameters!(Lexer)
libd-llvm/libd/src/d/parser/declaration.d(362):
instantiated from here: parseFunction!(FunctionDeclaration,
Lexer, string,Type)
libd-llvm/libd/src/d/parser/declaration.d(70): ... (2
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(46): Error: template
instance d.parser.dtemplate.parseTemplateParameter!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/dfunction.d(51): instantiated
from here: parseTemplateParameters!(Lexer)
libd-llvm/libd/src/d/parser/declaration.d(362):
instantiated from here: parseFunction!(FunctionDeclaration,
Lexer, string,Type)
libd-llvm/libd/src/d/parser/declaration.d(70):
instantiated from here: parseTypedDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/dmodule.d(40): ... (1
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dfunction.d(51): Error: template
instance d.parser.dtemplate.parseTemplateParameters!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/declaration.d(362):
instantiated from here: parseFunction!(FunctionDeclaration,
Lexer, string,Type)
libd-llvm/libd/src/d/parser/declaration.d(70):
instantiated from here: parseTypedDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/dmodule.d(40): instantiated
from here: parseDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/declaration.d(362): Error: template
instance d.parser.dfunction.parseFunction!(FunctionDeclaration,
Lexer, string,Type) error instantiating
libd-llvm/libd/src/d/parser/declaration.d(70):
instantiated from here: parseTypedDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/dmodule.d(40): instantiated
from here: parseDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/declaration.d(70): Error: template
instance d.parser.declaration.parseTypedDeclaration!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/dmodule.d(40): instantiated
from here: parseDeclaration!(Lexer)
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/dmodule.d(40): Error: template
instance d.parser.declaration.parseDeclaration!(Lexer) error
instantiating
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/base.d(14): Error: template instance
d.parser.dmodule.parseModule!(Lexer) error instantiating
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
src/sdc/sdc.d(54): Error: template instance
d.parser.base.parse!(Lexer) error instantiating
libd-llvm/libd/src/d/parser/ambiguous.d(126): Error: no property
'bar' for type 'Lexer', did you mean 'r'?
libd-llvm/libd/src/d/parser/statement.d(392): Error: template
instance
d.parser.statement.parseStatement!(Lexer).parseStatement.parseDeclarationOrExpression!(__dgliteral27,
Lexer) error instantiating
libd-llvm/libd/src/d/parser/statement.d(406): instantiated
from here: parseStatement!(Lexer)
libd-llvm/libd/src/d/parser/expression.d(652):
instantiated from here: parseBlock!(Lexer)
libd-llvm/libd/src/d/parser/dtemplate.d(208): instantiated
from here: parsePrimaryExpression!(Lexer)
libd-llvm/libd/src/d/parser/identifier.d(83): ... (11
instantiations, -v to show) ...
libd-llvm/libd/src/d/parser/base.d(14): instantiated from
here: parseModule!(Lexer)
src/sdc/sdc.d(54): instantiated from here: parse!(Lexer)
libd-llvm/libd/src/d/parser/statement.d(406): Error: template
instance d.parser.statement.parseStatement!(Lexer) error
instantiating
make: *** [bin/sdc] Erreur 1
Yes I added the bar on purpose in some heavily templated code.
You concentrate too much on theses people that want to ship code
without using it. They'll do it anyway.
And that make even more sense from the lib user perspective, as
having the compiler vomit kilometers of internals of a lib is
usually not helpful (this happen a lot with phobos).
