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authorEdward Z. Yang <ezyang@cs.stanford.edu>2015-05-19 14:57:42 -0700
committerEdward Z. Yang <ezyang@cs.stanford.edu>2015-05-19 14:57:42 -0700
commitc9349146ea74f0e074e287f29581e759cc5f8afd (patch)
tree856bd71bbf0381ecdff8689178566956a33fa0d9 /docs
parent25d1a716395e349736994759d1fcbb3721f3ee9f (diff)
downloadhaskell-c9349146ea74f0e074e287f29581e759cc5f8afd.tar.gz
Backpack docs: Clarifications from today's Skype call.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Diffstat (limited to 'docs')
-rw-r--r--docs/backpack/algorithm.pdfbin279771 -> 280399 bytes
-rw-r--r--docs/backpack/algorithm.tex44
2 files changed, 28 insertions, 16 deletions
diff --git a/docs/backpack/algorithm.pdf b/docs/backpack/algorithm.pdf
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+++ b/docs/backpack/algorithm.pdf
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diff --git a/docs/backpack/algorithm.tex b/docs/backpack/algorithm.tex
index f3828b2adc..106dcc23ad 100644
--- a/docs/backpack/algorithm.tex
+++ b/docs/backpack/algorithm.tex
@@ -118,8 +118,12 @@ $$
\end{figure}
Shaping computes a \I{Shape}, whose form is described in Figure~\ref{fig:semantic}.
-Initializing the shape context to the empty shape, we incrementally
-build the context as follows:
+A shape describes what modules a package implements and exports (the \emph{provides})
+and what signatures a package needs to have filled in (the \emph{requires}). Both
+provisions and requires can be imported after a package is included.
+
+We incrementally build a shape by starting with an empty
+shape context and adding to it as follows:
\begin{enumerate}
\item Calculate the shape of a declaration, with respect to the
@@ -740,8 +744,8 @@ key from the identifiers.
module B where
data T = S { baz :: Bool }
module C(bar, baz) where
- import A
- import B
+ import A(bar)
+ import B(baz)
-- A.T{ A.bar }, B.T{ B.baz }
-- NB: it would be illegal for the type constructors
-- A.T and B.T to be both exported from C!
@@ -806,8 +810,8 @@ The answer is no! Consider these implementations:
data A = A { foo :: Int, bar :: Bool }
module A(foo, bar) where
- import A1
- import A2
+ import A1(foo)
+ import A2(bar)
\end{verbatim}
Here, \verb|module A1| implements \verb|signature A1|, \verb|module A2| implements \verb|signature A2|,
@@ -864,31 +868,36 @@ equivalent to the shapes for these which should merge:
\subsection{Subtyping record selectors as functions}
\begin{verbatim}
- signature H(foo) where
+ signature H(A, foo) where
data A
foo :: A -> Int
- module M(foo) where
+ module M(A, foo) where
data A = A { foo :: Int, bar :: Bool }
\end{verbatim}
%
Does \verb|M| successfully fill \verb|H|? If so, it means that anywhere
a signature requests a function \verb|foo|, we can instead validly
-provide a record selector. This capability seems quite attractive
-but actually it is quite complicated, because we can no longer assume
-that every child name is associated with a parent name.
+provide a record selector. This capability seems quite attractive,
+although in practice record selectors rarely seem to be abstracted this
+way: one reason is that \verb|M.foo| still \emph{is} a record selector,
+and can be used to modify a record. (Many library authors find this
+suprising!)
-As a workaround, \verb|H| can equivalently be written as:
+Nor does this seem to be an insurmountable instance of the avoidance
+problem:
+as a workaround, \verb|H| can equivalently be written as:
\begin{verbatim}
signature H(foo) where
data A = A { foo :: Int, bar :: Bool }
\end{verbatim}
%
-This is suboptimal, however, as the otherwise irrelevant \verb|bar| must be mentioned
+However, you might not like this, as the otherwise irrelevant \verb|bar| must be mentioned
in the definition.
-So what if we actually want to write the original signature \verb|H|?
+In any case, actually implementing this `subtyping' is quite complicated, because we can no
+longer assume that every child name is associated with a parent name.
The technical difficulty is that we now need to unify a plain identifier
\I{AvailInfo} (from the signature) with a type constructor \I{AvailInfo}
(from a module.) It is not clear what this should mean.
@@ -910,8 +919,11 @@ Consider this situation:
import X(A(..)) -- ???
\end{verbatim}
-Should the wildcard import on \verb|X| be allowed? Probably not?
-How about this situation:
+Should the wildcard import on \verb|X| be allowed?
+This question is equivalent to whether or not shaping discovers
+whether or not a function is a record selector and propagates this
+information elsewhere.
+If the wildcard is not allowed, here is another situation:
\begin{verbatim}
package p where