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.

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:

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

Using -fprint-explicit-kinds makes GHC print kind-foralls and kind applications 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 :: T b
ghci> :set -fprint-explicit-foralls
ghci> :t MkT
MkT :: forall (b::k). T b
ghci> :set -fprint-explicit-kinds
ghci> :t MkT
MkT :: forall (k::BOX) (b:k). T b

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).

Set the minimum size of the heap to size. This option is equivalent to +RTS -Hsize, see Section 4.17.3, “RTS options to control the garbage collector”.

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”.