CODEWITHSUNDEEP
haskell
Gilgamesz
Gilgamesz

Reputation: 5063

Deducing type in Haskell

Let 

f :: a -> Int
f arg = 2

a is a type that will be deduced during compilation. Is it possible to learn how did Haskell deduce it?

Upvotes: 0

Views: 206

Answers (1)

chi
chi

Reputation: 116139

I'm not sure whether this is actually useful... but here is one way:

> import Data.Typeable
> let argType f x = let _ = f x in typeOf x

Example:

> let f :: a->Int ; f arg = 2
> argType f 'a'
Char
> argType f 1
Integer

The last example shows the actual type of 1, after GHC(i) defaulting.

A simpler alternative, working even in source files: when we have

foo (bar x) (baz y)

and we want to know the type of y, we can just replace it with

foo (bar x) (baz (asTypeOf _ y))

We will get an error such as

    • Found hole: _ :: Integer

which tells us the type of y. Downside: we have to revert the code back for it to compile.

As a more low-level alternative, compile with -ddump-simpl and observe the scary GHC Core which is being produced. There, every type argument is made explicit: we could read something like f @ Char 'a', where names could be possibly mangled a bit, e.g. Main.a$f @ GHC.Char 'a', but it should still be possible to understand what's going on.

Upvotes: 3

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