While developing, sometimes it is desirable to allow compilation to succeed even if there are type errors in the code. Consider the following case:
module Main where a :: Int a = 'a' main = print "b"
Even though a is ill-typed, it is not used in the end, so if
all that we're interested in is main it can be useful to be
able to ignore the problems in a.
For more motivation and details please refer to the HaskellWiki page or the original paper.
The flag -fdefer-type-errors controls whether type
errors are deferred to runtime. Type errors will still be emitted as
warnings, but will not prevent compilation.
At runtime, whenever a term containing a type error would need to be evaluated, the error is converted into a runtime exception. Note that type errors are deferred as much as possible during runtime, but invalid coercions are never performed, even when they would ultimately result in a value of the correct type. For example, given the following code:
x :: Int x = 0 y :: Char y = x z :: Int z = y
evaluating x will result in a runtime type error.
The flag -fdefer-type-errors works in GHCi as well, with
one exception: for "naked" expressions typed at the prompt, type
errors don't get delayed, so for example:
Prelude> fst (True, 1 == 'a')
<interactive>:2:12:
No instance for (Num Char) arising from the literal `1'
Possible fix: add an instance declaration for (Num Char)
In the first argument of `(==)', namely `1'
In the expression: 1 == 'a'
In the first argument of `fst', namely `(True, 1 == 'a')'
Otherwise, in the common case of a simple type error such as
typing reverse True at the prompt, you would get a warning and then
an immediately-following type error when the expression is evaluated.
This exception doesn't apply to statements, as the following example demonstrates:
Prelude> let x = (True, 1 == 'a')
<interactive>:3:16: Warning:
No instance for (Num Char) arising from the literal `1'
Possible fix: add an instance declaration for (Num Char)
In the first argument of `(==)', namely `1'
In the expression: 1 == 'a'
In the expression: (True, 1 == 'a')
Prelude> fst x
True