See also the --help, --version, --numeric-version,
and --print-libdir modes in Section 4.5, “Modes of operation”.
-v
The -v option makes GHC
verbose: it reports its version number
and shows (on stderr) exactly how it invokes each phase of
the compilation system. Moreover, it passes the
-v flag to most phases; each reports its
version number (and possibly some other information).
Please, oh please, use the -v option
when reporting bugs! Knowing that you ran the right bits in
the right order is always the first thing we want to
verify.
-vn
To provide more control over the compiler's verbosity,
the -v flag takes an optional numeric
argument. Specifying -v on its own is
equivalent to -v3, and the other levels
have the following meanings:
-v0Disable all non-essential messages (this is the default).
-v1Minimal verbosity: print one line per
compilation (this is the default when
--make or
--interactive is on).
-v2Print the name of each compilation phase as it
is executed. (equivalent to
-dshow-passes).
-v3The same as -v2, except that in
addition the full command line (if appropriate) for
each compilation phase is also printed.
-v4The same as -v3 except that the
intermediate program representation after each
compilation phase is also printed (excluding
preprocessed and C/assembly files).
--fprint-explicit-foralls, -fprint-explicit-kinds
These two flags control the way in which GHC displays types, in error messages and in GHCi.
Using -fprint-explicit-foralls makes GHC print explicit forall
quantification at the top level of a type; normally this is suppressed. For example, in GHCi:
ghci> let f x = x ghci> :t f f :: a -> a ghci> :set -fprint-explicit-foralls ghci> :t f f :: forall a. a -> a
However, regardless of the flag setting, the quantifiers are printed under these circumstances:
For nested foralls, e.g.
ghci> :t GHC.ST.runST GHC.ST.runST :: (forall s. GHC.ST.ST s a) -> a
If any of the quantified type variables has a kind that mentions a kind variable, e.g.
ghci> :i Data.Coerce.coerce
coerce ::
forall (k :: BOX) (a :: k) (b :: k). Coercible a b => a -> b
-- Defined in GHC.Prim
Using -fprint-explicit-kinds makes GHC print kind arguments
in types, which are normally suppressed. This can be important when you are using kind polymorphism.
For example:
ghci> :set -XPolyKinds ghci> data T a = MkT ghci> :t MkT MkT :: forall (k :: BOX) (a :: k). T a ghci> :set -fprint-explicit-foralls ghci> :t MkT MkT :: forall (k :: BOX) (a :: k). T k a
-ferror-spans
Causes GHC to emit the full source span of the syntactic entity relating to an error message. Normally, GHC emits the source location of the start of the syntactic entity only.
For example:
test.hs:3:6: parse error on input `where'
becomes:
test296.hs:3:6-10: parse error on input `where'
And multi-line spans are possible too:
test.hs:(5,4)-(6,7):
Conflicting definitions for `a'
Bound at: test.hs:5:4
test.hs:6:7
In the binding group for: a, b, a
Note that line numbers start counting at one, but column numbers start at zero. This choice was made to follow existing convention (i.e. this is how Emacs does it).
-Hsize
Set the minimum size of the heap to
size.
This option is equivalent to
+RTS -H,
see Section 4.17.3, “RTS options to control the garbage collector”.
size
-Rghc-timing
Prints a one-line summary of timing statistics for the
GHC run. This option is equivalent to
+RTS -tstderr, see Section 4.17.3, “RTS options to control the garbage collector”.