Commit 5c93df90 authored by Simon Peyton Jones's avatar Simon Peyton Jones

Improve comments on AbsBinds

See Trac #13827.
parent 986deaa5
......@@ -301,15 +301,57 @@ deriving instance (DataId idL, DataId idR) => Data (PatSynBind idL idR)
{-
Note [AbsBinds]
~~~~~~~~~~~~~~~
The AbsBinds constructor is used in the output of the type checker, to record
*typechecked* and *generalised* bindings. Consider a module M, with this
top-level binding, where there is no type signature for M.reverse,
The AbsBinds constructor is used in the output of the type checker, to
record *typechecked* and *generalised* bindings. Specifically
AbsBinds { abs_tvs = tvs
, abs_ev_vars = [d1,d2]
, abs_exports = [ABE { abe_poly = fp, abe_mono = fm
, abe_wrap = fwrap }
ABE { slly for g } ]
, abs_ev_binds = DBINDS
, abs_binds = BIND[fm,gm] }
where 'BIND' binds the monomorphic Ids 'fm' and 'gm', means
fp = fwrap [/\ tvs. \d1 d2. letrec { DBINDS ]
[ ; BIND[fm,gm] } ]
[ in fm ]
gp = ...same again, with gm instead of fm
The 'fwrap' is an impedence-matcher that typically does nothing; see
Note [ABExport wrapper].
This is a pretty bad translation, because it duplicates all the bindings.
So the desugarer tries to do a better job:
fp = /\ [a,b] -> \ [d1,d2] -> case tp [a,b] [d1,d2] of
(fm,gm) -> fm
..ditto for gp..
tp = /\ [a,b] -> \ [d1,d2] -> letrec { DBINDS; BIND }
in (fm,gm)
In general:
* abs_tvs are the type variables over which the binding group is
generalised
* abs_ev_var are the evidence variables (usually dictionaries)
over which the binding group is generalised
* abs_binds are the monomorphic bindings
* abs_ex_binds are the evidence bindings that wrap the abs_binds
* abs_exports connects the monomorphic Ids bound by abs_binds
with the polymorphic Ids bound by the AbsBinds itself.
For example, consider a module M, with this top-level binding, where
there is no type signature for M.reverse,
M.reverse [] = []
M.reverse (x:xs) = M.reverse xs ++ [x]
In Hindley-Milner, a recursive binding is typechecked with the *recursive* uses
being *monomorphic*. So after typechecking *and* desugaring we will get something
like this
In Hindley-Milner, a recursive binding is typechecked with the
*recursive* uses being *monomorphic*. So after typechecking *and*
desugaring we will get something like this
M.reverse :: forall a. [a] -> [a]
= /\a. letrec
......@@ -326,19 +368,22 @@ That's after desugaring. What about after type checking but before
desugaring? That's where AbsBinds comes in. It looks like this:
AbsBinds { abs_tvs = [a]
, abs_ev_vars = []
, abs_exports = [ABE { abe_poly = M.reverse :: forall a. [a] -> [a],
, abe_mono = reverse :: [a] -> [a]}]
, abs_ev_binds = {}
, abs_binds = { reverse :: [a] -> [a]
= \xs -> case xs of
[] -> []
(x:xs) -> reverse xs ++ [x] } }
Here,
* abs_tvs says what type variables are abstracted over the binding group,
just 'a' in this case.
* abs_tvs says what type variables are abstracted over the binding
group, just 'a' in this case.
* abs_binds is the *monomorphic* bindings of the group
* abs_exports describes how to get the polymorphic Id 'M.reverse' from the
monomorphic one 'reverse'
* abs_exports describes how to get the polymorphic Id 'M.reverse'
from the monomorphic one 'reverse'
Notice that the *original* function (the polymorphic one you thought
you were defining) appears in the abe_poly field of the
......@@ -537,32 +582,6 @@ plusHsValBinds (ValBindsOut ds1 sigs1) (ValBindsOut ds2 sigs2)
plusHsValBinds _ _
= panic "HsBinds.plusHsValBinds"
{-
What AbsBinds means
~~~~~~~~~~~~~~~~~~~
AbsBinds tvs
[d1,d2]
[(tvs1, f1p, f1m),
(tvs2, f2p, f2m)]
BIND
means
f1p = /\ tvs -> \ [d1,d2] -> letrec DBINDS and BIND
in fm
gp = ...same again, with gm instead of fm
This is a pretty bad translation, because it duplicates all the bindings.
So the desugarer tries to do a better job:
fp = /\ [a,b] -> \ [d1,d2] -> case tp [a,b] [d1,d2] of
(fm,gm) -> fm
..ditto for gp..
tp = /\ [a,b] -> \ [d1,d2] -> letrec DBINDS and BIND
in (fm,gm)
-}
instance (SourceTextX idL, SourceTextX idR,
OutputableBndrId idL, OutputableBndrId idR)
=> Outputable (HsBindLR idL idR) where
......
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