# NOTE: simply typed lambda calculus

Last time I introduce lambda calculus. Lambda calculus is powerful enough for computation. But it's not good enough for people, compare with below Church Numerals

$add := \lambda m. \lambda n. \lambda s. \lambda z. m\;s\;(n\;s\;z)$

people prefer just + more.

But once we introduce such fundamental operations into the system, validation would be a thing. This is the main reason to have a $\lambda \to$ system(a.k.a. simply typed lambda calculus). It gets name $\lambda \to$ is because it introduces one new type: Arrow type, represent as $T_1 \to T_2$ for any abstraction $\lambda x.M$ where $x$ has a type is $T_1$ and $M$ has a type is $T_2$. Therefore we can limit the input to a specified type, without considering how to add two Car together!

To represent this, syntax needs a little change:

term ::=                                                     terms
x                                                           variable
λx: T.term                                     abstraction
term term                                     application


Abstraction now can describe it's parameter type. Then we have typing rules:

$\frac{ x:T \in \Gamma }{ \Gamma \vdash x:T } \;\;\;\; T-Variable \\ \frac{ \Gamma, x:T_1 \vdash t_2: T_2 }{ \Gamma \vdash \lambda x:T_1.t_2 : T_1 \to T_2 } \;\;\;\; T-Abstraction \\ \frac{ \Gamma, t_1:T_1 \to T_2 \; \Gamma \vdash t_2: T_1 }{ \Gamma \vdash t_1 \; t_2 : T_2 } \;\;\;\; T-Application$

Here is explaination:

• T-Variable: with the premise, term $x$ binds to type $T$ in context $\Gamma$ is truth. We can make a conclusion, in context $\Gamma$, we can judge the type of $x$ is $T$.
• T-Abstraction: with the premise, with context $\Gamma$ and term $x$ binds to type $T_1$ we can judge term $t_2$ has type $T_2$. We can make a conclusion, in context $\Gamma$, we can judge the type of $\lambda x:T_1.t_2$ is $T_1 \to T_2$.
• T-Application: with the premise, with context $\Gamma$ and term $t_1$ binds to type $T_1 \to T_2$ and with context $\Gamma$ we can judge term $t_2$ has type $T_1$. We can make a conclusion, in context $\Gamma$, we can judge the type of $t_1 \; t_2$ is $T_2$.

Date: 2020-03-08 Sun 00:00