Difference between revisions of "Type signature"

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</haskell>
 
</haskell>
 
that tells, what is the [[type]] of a [[variable]].
 
that tells, what is the [[type]] of a [[variable]].
In the example <hask>inc</hask> is the variable and <hask>Num a => a -> a</hask> is its type,
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In the example <hask>inc</hask> is the variable, <hask>Num a =></hask> is the context and <hask>a -> a</hask> is its type, namely a [[function]] type with the kind <hask>* -> *</hask>.
namely a [[function]] type.
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  +
A very simple example looks like this
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<haskell>
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title :: String
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</haskell>
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which restricts the variable <hask>title</hask> to the the type <hask>String</hask>. Binding a value of any other type will lead to a type missmatch. For example binding <hask>42</hask> to <hask>title</hask> by writing <hask>title = 42</hask> will lead to an error looking like this in ghc 7.10
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<haskell>
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No instance for (Num String) arising from the literal ‘42’
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In the expression: 42
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In an equation for ‘title’: title = 42
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</haskell>
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To better understand the error message, take a look at the types, in ghci you can use the `:t ` command which will show you the type for a given expression
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<haskell>
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>:t title
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title :: String
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>:t 42
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42 :: Num a => a
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</haskell>
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If instead declaring the types like in this example
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<haskell>
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title :: String
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value :: Integer
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value = 42
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title = value
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</haskell>
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the error message becomes clearer
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<haskell>
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Couldn`t match type ‘Integer’ with ‘[Char]’
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Expected type: String
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Actual type: Integer
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In the expression: value
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In an equation for ‘title’: title = value
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</haskell>
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but there is still room for confusion because the first lines mentiones the type `[Char]` which does not appear in the type signatures in the example. This comes form the fact that <hask>String</hask> is just a renaming for <hask>[Char]</hask>. The compiler only typechecks the expressions after resolving the renaming.
   
 
It is considered [[Type signatures as good style|good style]] to add a type signature to every [[top-level variable]].
 
It is considered [[Type signatures as good style|good style]] to add a type signature to every [[top-level variable]].

Latest revision as of 10:49, 8 November 2016

A type signature is a line like

inc :: Num a => a -> a

that tells, what is the type of a variable. In the example inc is the variable, Num a => is the context and a -> a is its type, namely a function type with the kind * -> *.

A very simple example looks like this

title :: String

which restricts the variable title to the the type String. Binding a value of any other type will lead to a type missmatch. For example binding 42 to title by writing title = 42 will lead to an error looking like this in ghc 7.10

    No instance for (Num String) arising from the literal 42
    In the expression: 42
    In an equation for title: title = 42

To better understand the error message, take a look at the types, in ghci you can use the `:t ` command which will show you the type for a given expression

>:t title
title :: String
>:t 42
42 :: Num a => a

If instead declaring the types like in this example

title :: String
value :: Integer

value = 42
title = value

the error message becomes clearer

    Couldn`t match type Integer with [Char]
    Expected type: String
      Actual type: Integer
    In the expression: value
    In an equation for title: title = value

but there is still room for confusion because the first lines mentiones the type `[Char]` which does not appear in the type signatures in the example. This comes form the fact that String is just a renaming for [Char]. The compiler only typechecks the expressions after resolving the renaming.

It is considered good style to add a type signature to every top-level variable.

References